Chipamp Dual Mono LM3886 Gainclone Speaker Amp

I'll be dipping my feet in the chip amp waters again. I enjoyed the results of my previous Chipamp.com LM3886 build, so I decided to build another, more serious version. You can visit that post to read a little bit about the history of the chipamp, also known as the Gainclone, based on 47Labs Gaincard topology.

As you are likely aware, DIYers have created chip-based builds using a variety of power opamp or audio opamp chips including National Semiconductor LM1875, LM3875, LM3886, and LM4780. The most popular chips being the LM3886 and LM3875. You'll find a number of these designs at Chipamp Electronics.

This more serious build features a pair of Antek 300VA torroids, each with their own shield, integrated amp functionality with an Alps Blue Velvet and multiple selectable inputs, film capacitors on the driver board, and higher end parts like Nichicon Gold Tune power caps, Cardas binding posts, and Cardas RCA connectors.

Cardas wire, RCA connectors and binding posts

Other nice additions to the build include a 6 position Grayhill selector switch and the Alps Blue Velvet RK27 (100K) volume potentiometer, both with the corresponding ChipAmp BrianGT PCBs. The RK27 board is handy as you don't have to discern which pin is which (the white screen tells you which wire goes where). It's also a little easier that soldering to the little pins. The Grayhill switch's pins are so minuscule that you really have to use the board, it's not like an Elma or Goldpoint with larger gold solder lugs. The nice thing is the combo is a fraction of the price of the fancier selector switches.

  

Chipamp's PCBs for pre-amps / integrated amps

This project would require a larger box than the Bud 7" x 12" x 3" aluminum box I used previously. Rather than build my own gravity mount chassis, I wanted something a little different this time. I turned to Horace Atkinson of www.iagaudio.com for a robust 17.5" W x 10"D x 3.75"H chassis. I stumbled upon his chassis work on eBay: it's a nice combination of aluminum and wood panels for sides, as well as attractive wood accents on the bottom. The metal used in these chassis is a thicker gauge, one piece .125" aluminum tubing, so it can support heavy transformers without any bend or flex. (FYI IAG offers thicknesses up to .188"). The bottom features perforated sheet metal for adequate air ventilation reaching the heatsinks cooling the LM3886. A unique feature is the wood side panels actually act as feet, holding the aluminum portion up higher to allow for ventilation, very cool!

IAG Audio aluminum and wood chassis 

A view of the bottom with the perforated sheet metal removed. Note the bottom of the chassis is not completely open due to the rigid, tubular design, however underneath the wood side panels, there are removable metal panels that can be removed by unscrewing two flush mounted screws to give you adequate access for drilling, etc.

Chassis interior

As noted, the wood panels are all easily removable if you decide you want to access the sides of the chassis interior or change the wood stain, etc. Horace found some beautiful figured maple for me, as you can see below:

Figured maple side panels and accents

After much internal debate, I decided the chassis would be colored to match some of my other equipment with a red and cream color combo, so the wood was stripped down and stained a deep red hue, then given a number of generous coats of MinWax Satin Clear Urethane.  

Red stain and urethane on Maple

Now that the wood is my a nice shiny red, it's time to drill the chassis. After planning out the layout in Photoshop, the chassis is prepped with a ruler and sharpie and taken to the drill press. Many of the holes will be countersunk for this project. The others will be deburred using a Skaviv deburring tool with Cobalt bits. The IEC hole is trimmed using the typical combo of drilling four holes in the corners and using a Dremel cutting wheel to remove the rest. 

Chassis prep

For the heatsinks, I found some reasonably priced ones on eBay that had mounting holes for screws. The instruction manual from Chipamp says you can also use a 3"x3"x1/2" piece of aluminum in free air in certain applications. I installed 1" aluminum standoffs on the heatsinks so I could mount them to the chassis. Small drilled venting holes will be above them so when heat rises it can escape the chassis. 

Below is the chassis after a nice cream colored powdercoat. 

Since the amp is dual mono, it gets not one, but two chunky toroids. The transformers are always what weigh down an amp (big magnets with large quantities of copper wire spun around them aren't the lightest of materials). The pair, with steel cases, weigh in at a healthy 22 pounds. These are 300VA rated units from Antek.

Below is one of the toroids in a steel case. I had these powdercoated brick red as it makes a nice accent color for the cream. Typically the steel cases are used as a shield within a chassis, however I'm using them as both a shield and external transformer covers. I don't think I've ever seen a solid state amp with external transformers (usually they're just hulking boxes) so this will be a unique design choice.  

I'm adding the Power Supply Soft Start Board (or SSB for short) from DIYAudio for this build, this allows a gentle inflow of power on startup, which protects the power caps and should offer a more reliable service life for the amp. 

With everything ready, the chassis can start to get filled in. Shiny Cardas RCAs adorn the back of the unit, the binding posts would have been a bit tight on the back, so they were mounted to the top of the chassis right behind the transformers. Four large film caps will be residing in this chassis as well, so you'll notice some zip ties within the chassis as well.

Getting everything in place was a tricky predicament, as the sides of the chassis which held the heatsinks were screwed in place, then the wood panels were screwed to them, but the capacitors would have to be slid in to be held and secured, so order of operations took precedent. 

The power capacitors were a bit tall, so I had to use some minuscule PCB standoffs so they wouldn't come in contact with the chassis. The film caps, as you can see, were a perfect fit. I made a few small turret boards to keep the wiring neat. Cardas wiring handles all the signal work and thicker power wires as well. If you look carefully you'll notice one of the blue wires exists after the potentiometer to the RCAs to handle subwoofer duty. 

And the final product, see what you think:

Final weight was about 40 lbs. thanks to the hefty transformers and very solid tubular chassis. It's a unique take on a solid state amp, not the typical big black box with heatsinks on the side, but a more vintage tube-amp looking chassis. The sound is excellent, will be looking forward to putting it through the burn-in process and most more impressions.

The Fine Print:

Please remember that building/modifying circuits can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind. 

Bottlehead Fix for Full Range Speakers

The Bottlehead Fix is a passive baffle step corrector and treble compensating device that sits between your source and your integrated amplifier, or between your pre-amp and amp. Baffle step correction (if the baffle is narrow) and treble compensation can benefit the sound of full range speakers in particular, so if you are into Fostex or MarkAudio drivers (or even Planet10 / Feastrex modified drivers), this is a great device to check out. Do note, in some cases, the compensation is already being handled by a network of capacitors, inductors and resistors within a full-range speaker.

There are four switches in total. As stated on the Bottlehead website: two of switches on the Fix controls the adjustable baffle step corrector that compensates for the bass roll-off in narrow baffled speakers. The first switch is used to set the bass turn-over frequency for the baffle width of the speaker you are using (selectable in inches). The second switch controls magnitude of the cutoff frequencies for passive bass compensation (which flattens a falling frequency response below baffle step frequency).

The third and forth switch compensate for treble roll-off with typical full range drivers. One switch selects turnover frequency, and the second switch chooses 0,2,or 4 dB of treble compensation above that frequency.

Curious if your speakers could benefit or not? The Fix is only $75 so it's not a big gamble, and given the popularity of full range speakers lately it wouldn't be a tough sell either. 

Normally, the Fix comes with a random colored piece of plexiglass (or similar material) and you either make the base yourself, or order one of Hammond or Bottlehead's for a few extra bucks. The build should be pretty quick as there aren't many parts, and would take the average person a couple of short nights to put together. 

Since I like everything on my rack to be "matchy matchy", I'll be using some alternative materials to put mine together, namely the typical copper leafed wood base and a copper plate. The copper plate I ordered was a remnant online, nice and heavy at 1/8" thick. I taped the plexi plate to the copper one and used it as a drilling template on the drill press. 

After drilling the holes, I sanded with 60, 180, 300 and 600 grade sandpaper, then spray painted it with a matte clearcoat from Krylon, otherwise it would be oxidized with hand-prints the very next day. 

Drilled copper plate

The base is made from 2 inch tall decorative molding from the local hardware store, cut with a miter saw, adhered with wood glue, and braced in the corners with square pieces of pine. It's then copper leafed using generic copper leaf and Mona Lisa Speedball base coat, adhesive size, sealer and antiquing glaze. These items can be found at your local Michaels craft store or online. 

Sitting in a copper leaf base

The first step in assembling the Fix is fastening on the switches and RCA female connectors. A socket wrench is especially handy in this step, however if using the plexi base, caution is advised in the manual as it can crack. In addition, the switches have plastic threading, so they could strip easily if too much pressure is used. 

Switches mounted

Once these items are secure, it's then time to mount the solder tabs. The manual shows corresponding letters and numbers for each solder point, so it's great to write these directly on the plate for ease of assembly. 

Labeling the solder points

I ran a solid piece of Cardas magnet wire for the ground plane and tied any solder tabs that needed to be grounded to it using bus wire. The polystyrene caps supplied are very nice quality. Polystyrene was commonly used for capacitors in the 50s up through the late 70s. You'll still find them today, many of them surplus from overseas. In vintage tube amps you may find them in glass-sealed tubes. This variety has the leads laid-in, and you need to be careful in higher values as they will create inductance based on the way they are built. Also polystyrene caps can melt, so don't put them in areas of high temperature! Those are the cons, but many feel that polystyrene caps are second only to Teflon in sound quality, so it may be worth it to experiment with them.

The 15,000pF caps in the Bottlehead kit are polypropylene, I opted to order some polystyrene to match the rest of the build. These came from Israel and are a little larger as they are 125V (no particular reason, just the smallest value I could find).

I opted to use some Cardas hookup wire and carbon film resistors for the build, just based on my own preferences. The ethernet wire that Bottlehead supplies is extremely high quality, so go with that unless you are a die-hard Cardas fan ;)

Bottlehead Fix interior

With the 15000pF caps installed

Bottlehead supplies a nice piece of black vinyl that has the functions printed on it, as well as an "I" and "O" for input and output. As a final touch, I added the iconic Bottlehead badge to the lower center.   

The completed Bottlehead Fix

I hope you enjoyed this post. Please visit Zynsonix if you need some sweet sounding interconnects for your Bottlehead gear, or any other custom audio cable. Until next time! 

The Fine Print:

The above steps detailing the build of a Bottlehead Fix are for entertainment purposes only, and not intended to be used as assembly instructions. Zynsonix Audio LLC is in no way affiliated with Bottlehead. The owner of this blog and all associated parties can not / will not be held responsible if you attempt the process posted and cause physical harm to yourself, your surroundings or your property. 

A Customized Dynaco ST35 Kit

The ST35, also known as the Stereo 35, was released by Dynaco in 1963. It's the baby brother of the Dynaco ST70 and tends to be overlooked by many vintage amplifier aficionados. I think the main reason this is the case is that there isn't a large price difference between the two units, and with a tube rectified power supply, power choke, bias system and twice the wattage, it's hard not to want to step up to the ST70. For those of us with relatively sensitive speakers, the ST35 is a nice little unit with a small footprint and more than ample power (17.5 watts x 2). I've also read from more than one source that the ST35 is the best sounding of the Dynaco units stock, for whatever that's worth ;)

Because there are a lot less ST35s floating around, there aren't really any boards with modern circuits that fit into the original chassis. Shannon Parks of DIYTube has created a brand new circuit board that is similar to the ST35 circuit with some modern sensibilities if one would like to go that route. I prefer the looks of the original ST35 chassis myself, with the exposed PCBs on the ends. For this, I turned to Dynakit Parts. Dynakit Parts is a company based in Clifton, New Jersey and has an impressive selection of old school reproductions of Dynaco products. The kits are about as close to the originals as you can get, from the can capacitor to the screw-down terminal strips.

Dynakit Parts ST35 Kit

The original Dynaco ST35 Schematic

Rather than purchasing the whole kit, I purchased a number of the parts a la carte, as I was interested in adding a little bit of customization to the kit with a power choke, power switch, triode / ultra-linear switches, a C8 power inlet for a custom C7 removable power cord and a number of boutique parts. I also added the ST35-BCU bias control upgrade kit which fits nicely above the chassis. As stated on the Dynakit site:  The ST35-BCU employs an adjustable fixed bias circuit which allows for user adjustment of the bias setting of each (EL84) output tube via (4) separate on board precision bias pots. This allows you to not have to purchase a matched set of power tubes. And even if you did purchase a matched quad, power tubes tend to age differently, so they can be adjusted after this occurs. 


Kevin of Dynakit Parts was working on a brand new brown PCB board for the ST35 when I contacted him. I really appreciate it when a company isn't resting on their laurels but investing in new products and innovations. The new PC-13 boards from Dynakit is made from aerospace grade 240 degree Centigrade brown Polymide mil spec material and features silk screened component designations. The brown will go very nicely with the vintage patina powdercoat scheme that I had planned. I wish the bias board was made of the same nice brown material, but we can't have it all, can we? ;)

Brown Polymide boards with a few parts in place

I elected to populate the small driver / power boards with Kiwame carbon film resistors, Obbligato and Mundorf Supreme film capacitors and Teflon and ceramic tube sockets (the Teflon sockets even have beryllium copper contacts). The caps fit pretty much perfectly, although the Mundorfs will have to be mounted once the boards are mounted due to sizing constraints. The Dynakit build calls for nicer silver mica capacitors in place of the ceramic disc capacitors that were in the older units. These can be found just about anywhere (Mouser, Digikey, Partsconnexion, Handmade, etc.). 


I chose to go with the cadmium metal chassis as opposed to the newer stainless steel version as the chassis is going to be powder-coated; no need to pay extra for that chromed-out finish when no one's going to see it ;) Below you will see the chassis, I've enlarged the holes to fit the Cardas binding posts and RCAs, and the center hole has been enlarged to hold a piece of FR4 material that will then hold the C8 inlet in place. I then gave the chassis a nice sanding and labelled it for the powder-coater. 

A few chassis mods and a bit of sanding prep

One thing I would be painting myself was the power choke. A choke is a nice cheap upgrade for the ST35 and helps reduce ripple in the power supply. A number of people have used the C354 to replace the 50 ohm resistor, I chose a Hammond 156R with 56ohms DCR, which is pretty much the equivalent. Chokes tend to be on the ugly side, so if they aren't hiding in the chassis (there's absolutely no room in this case), they need to be dressed up a bit. I was prepping two at once, one for the ST35 and one for the Millett Jonokuchi. 


The first step in this process is to sand down the metal frame on the choke. Then painters tape is cut and applied to mask off the inside. I've seen some builders just paint the whole thing, but I don't really care for that look. 

Chokes masked off with painters tape

Next I apply two layers of primer, one per hour, then multiple coats of Hammertone paint. It can be difficult to get the Hammertone paint to lay correctly without making the coat too thick. Any over-spray is corrected using black permanent ink. 

Chokes painted with copper Hammertone paint

Once dry, I measured and cut some strips of black leather and used some epoxy to keep them in place, wrapping around the spool of the choke. I use a couple layers of leather when possible to give the spool a fuller look, it's more aesthetically pleasing.

Completed chokes with leather wrapping the spool. 

UPDATE (6/19/11): The parts arrived for the ST35 power / driver boards, so I went ahead and populated everything except for C4 and C5, which will be 0.1uF Mundorf Supremes that will hang off the board just a bit and will need to be installed once the boards are fastened to the chassis. C1 was not used per Bob Latino's suggestion as its purpose is DC input blocking and nearly all modern-day preamps already have a DC blocking output cap, so I used a pair of thick leads to short C1. As you can see below, the Teflon tube sockets have blue PCB boards installed to ease the soldering process. The sockets had a rather long ground pin that I trimmed down and sanded smooth as there would have been a clearance issue in the height challenged chassis. 

ST35 boards mostly populated and ready for installation

UPDATE (7/8/11): The chassis is pack from the powder-coater so I was able to fit the boards in place. I opted to install them rotated 180 degrees as I wanted the RCAs on the rear of the chassis and there wasn't sufficient clearance with the power tube sockets blocking the way. I took some generic gold-plated RCAs from my parts bin and trimmed off the back with a cutting disc, rendering them nonfunctional but small enough to squeeze between the tube socket and the chassis front, simply for the sake of aesthetics. I was also able to position the painted Hammond filter choke, attach the Cardas copper binding posts and install the Cardas rhodium plated RCA jacks. Also installed was the large silver multicap and the bias control board from Dynakitparts. 

Getting started assembling the parts on the chassis

In the interest of making things convenient, I installed a small 6A SPST toggle switch on the front panel of the chassis so the amp can be powered on and off conveniently.

A 3 Amp power switch installed on the front of the unit

The ST35 Bias Control Unit from DynakitParts makes use of an adjustable fixed-bias circuit, allowing for adjustment of the bias setting of each EL84 power tube via four separate precision bias potentiometers. 

Wires run to the Bias Control Unit from DynakitParts

The initial wiring was completed using Kimber TCSS 19 gauge stranded copper in teflon dielectric. I opted to drill out the area originally designated for the fuse holder and the cable in the interest of fitting a C8 power inlet. Because the re-purposed real estate on the chassis back was a little too large for the C8 inlet, I mounted it to a piece of FR-4 material and then mounted the combo to the chassis. This adds a degree of convenience, allowing a removable C7 power cord to be used to power the amp. The fuse was then moved inside the unit and mounted to one of the chassis braces, really a perfect place for it in my humble opinion. Grounding lugs were added to both chassis braces to connect to the RCA grounds (the Cardas RCAs are insulated from the chassis unlike the stock Dynaco RCAs) and the negative connector of the speaker binding posts. 

Initial wiring of the ST35 complete

UPDATE (7/28/11): The transformer bell-ends arrived back from the powdercoater and were reattached to the iron with brass hardware. Rather than using the cut transformer wire, I prefer the Kimber TCSS Teflon wire for all extraneous point-to-point wiring. The dielectric doesn't melt, it's a little thinner so it's easier to route, and I'd imagine the copper's of a higher purity. I opted to add a set of three 1uF 630V Solen film capacitors as bypass caps for the can capacitor. The Solen film caps are only a couple bucks a pop and have the potential to add refinement to the power supply. It's a narrow fit in the ST35 case, no room for zip tie mounts for the caps. 

Transformers wired in place, Solen bypass caps added

The wiring was finalized by adding a pair of DPDT switches to manage the triode and ultralinear switching. This was wired based on the following advise from Bob Latino and Tazsmonn on the DynacoTubeAudio Forum: Remove green or green white wire from pin #9 of each tube and solder to separate poles of DPDT switch on the same end, Solder wire from pin #9 to center pole of DPDT switch, Solder 100 ohm [resistor] from pin #7 to DPDT switch opposite end from green or green white wire, All wiring to and from switch side must be to same tube.

Triode mode switches wired up

Now that the wiring is complete, a final checkout of the circuit is performed, tubes and installed, the unit is powered on and the tubes biased using the ST35-BCU. The tubes installed were Mullard EL84s and Phillips EGC 12DW7. The sound is quite excellent, very resolving and holographic imaging. It actually sounds quite "hifi" for a circuit from the early 1960s. I could use a tiny bit more bass, but I was listening on a small set of Tektons with 4" Fostex full range drivers. Here's some additional progress pics:

Dynaco ST35 front view

Final Dynaco ST35 back view

Dynaco ST35 side view

UPDATE 8/8/2011: I was able to have a pair of custom engravings made for the unit. A 1" gold circle with the word "Dynaco" engraved in black and a small rectangle with a Dynaco Stereo 35 logo that I made in Adobe Illustrator. These are the final touch.

Final Dynaco ST-35 front view

Final Dynaco ST-35 top view

Engraving detail on quad-cap

Final Dynaco ST-35 rear view

To recap, the customizations to the kit were as follows:

• Teflon tube sockets with Beryllium copper contacts
• Ceramic gold plated driver tube sockets
• Mundorf and Obbligato driver board capacitors
• Solen power bypass caps
• Kimber TCSS internal wiring
• Kiwame / Koa Speers carbon film resistors
• Hammond 156R Choke - 56ohms DCR
• Dynakit Parts ST35 BCU (Bias Control Unit)
• Front power switch
• Triode / Ultralinear Switches
• Cardas RCAs and Binding Posts
• C8 power inlet jack with custom C7 power cord
• Custom powdercoat color-scheme
• Alternate fuse holder and placement
• Custom Brass Engravings

The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.