Modifying the MHDT Paradisea NOS DAC

The MHDT Paradisea is an excellent sounding example of a NOS (non over-sampling) tube DAC. If you're not already familiar with MHDT's offerings or the concept of a NOS DAC, I encourage you to read through my post regarding the modification of the MHDT Constantine DAC. Both of the units make use of a Philips TDA 1545A DAC chip and come in a handsome translucent dark acrylic.

The main difference between the two units is the 5670 tube buffer, which is directly coupled to the op-amp. Per the Enjoy the Music review of the Paradisea, various tube equivalents can be substituted, such as the 2C51, Western Electric 396A, 6N3Pi (6H3pi), or the 6CC42. I have yet to experiment much with tube rolling on the unit, but the example here came with a Western Electric 396A. Visibly, there are quite a few more parts in the Paradisea, and MHDT appears to have used mostly Nichicon MUSE electrolytics versus the Elna Silmics in the Constantine. The Paradisea also contains a torroid rather than a flat-pack transformer, it appears that MHDT has switched their entire line to torroids now, so I'd imagine this difference is indicative of the vintage of the unit. 


Of course the unit was going to be going under the knife, but I wanted to identify my purpose for doing so. When I first plugged in the Paradisea, I immediately noticed a more crisp and detailed presentation as compared to the Constantine. While the added detail is nice, the treble energy may be just a tad much for those of us with sensitive ears. Taking a look at the parts inside, every resistor was blue, which in nearly all cases means that they are metal film. Carbon film resistors typically are a tan color, of course when it comes to boutique resistors, vendors color them whatever they like (pink Takmans, green Kiwames, red PRPs, etc.).

The reason this makes a difference is that carbon films generally have a slightly warmer and more natural sound compared to the more detailed sound of metal film. The vast majority of new equipment makes use of metal film as it's very easy and cheap to get values within 1%. Carbon film is more typically found in vintage equipment, along with the slightly sweeter sounding carbon comp resistors. People tend to stray away from carbon composition these days (except for grid stoppers) as they pick up a little more noise and can drift over time, but they do have a nice sound and can be used as long as you are mindful of their shortcomings. Because I'm more fond of the sound characteristics of carbon film versus metal film, I decided that I wanted to replace as many metal film resistors as I could from the Paradisea. 

The original MHDT Paradisea with the top off

Resistors cannot be measured reliably when within a circuit, they need to be removed to be measured or you can read the color stripes on the resistor, although this can sometimes be difficult to determine a color (orange versus red, white versus gray) on a minuscule resistor. In most cases, I removed the resistor to measure it, wrote down the value and position, and only if it was a strange 1% value I put it back. Any other resistors that visibly matched these values were not removed. Common values were replaced with a combination of Kiwame (Koa Spears) and Takman resistors, the Kiwame leads needing a little bit of filing to fit. Some 1% uncommon values were replaced with generic 5% carbon films; I basically purchased large amounts of close values and measured them, picking out the ones that measured within 1% of the color code on the original resistor. It's a very slow and regimented process, but is enjoyable if you like working on electronics ;) 

Modifications in progress

The MHDT units come with some very nice parts for sure, but there can always be room for minor improvements. I opted to replace the stock film caps, which are relatively small compared to other caps of the same value. Based on dimensions, Auricaps, Obbligatos or ClarityCaps would be the closest match, all being a little bit larger. I selected Obbligatos as they tend to be fairly neutral and are a nice price for what you get. The problem with Obbligatos is their attractive gold shell is conductive, so when the leads are bent around the bottom or the cap is sitting on solder eyelets, the shell will short them. I usually put some 3:1 clear heatshrink over top to prevent this from happening. 

Clear heatshrink over top of the Obbligato cap

Heatshrink shrunk to fit

For the output caps, which is a very audible position for capacitors, I selected a matched pair of 2.2uF Mundorf Silver in Oil. These are quite large, with thick leads, so some creative mounting was required. The solder eyelets for the capacitor leads had to be enlarged with a 1/16 drill bit, and one of the caps actually sits outside of the unit for a very hot rodded look :) When solder eyelets are drilled, it effectively separates the conductivity from the top and the bottom of the board, so in some circumstances, depending on whether the copper traces on the PCB run along the top or the bottom, you have to solder not only to the bottom but the top of the board as well. 

Finished PCB - front view

Finished PCB - rear view

I opted to cover the leads of the right channel Mundorf capacitor with teflon, then sleeve them with black nylon as they would be visible from outside of the unit and needed to look nice. 

Finished mod, PCB in case with top off

Completed Modified Paradisea

The Paradisea now has a more warm, natural sound with slightly more bass quantity (while still being very controlled). Midrange seems to be more liquid and smooth. Overall, I'm very happy with the presentation. Now for some tube and opamp rolling! :) 


UPDATE (6/11/11): The Paradisea is made for op-amp rolling (pulling out the stock operational amplifier and putting a similarly spec'd one into the gold 8 pin socket). I dropped in a vintage NOS Tung Sol 2C51 tube and a Texas Instruments OPA2107AP op-amp in the unit. Immediately noticeable was a big bump in upper-end clarity and detail. I'll need to spend some time with the unit to see if this is preferable to the stock op-amp with Western Electric 396A or not (verify if this new-found detail is fatiguing at all over long periods).

The Fine Print:

Please remember that 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 posted modification 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.  

An Elma Stepped Attenuator Populated with Kiwame Resistors

Typically inexpensive kits come with a generic potentiometer to control volume, maybe something like an Alpha or an Alps Blue Velvet if you're lucky. A potentiometer is basically a is a three terminal resistor with a sliding contact. In a typical amp, the input signal runs directly through it and it references the signal to ground at a percentage determined by how much the pot is turned one way or the other. Pots are made using a resistive element formed into a circular arc, and a sliding wiper traveling over the arc. The resistive element is usually made from carbon, cermet or graphite.

If you skimp on the pot, you'll likely end up with something that may not be properly matched between left and right, you may experience static-like noise when turning it, and it likely won't sound all that great either. Really, a $14 Alps is the cheapest potentiometer I will go with on a normal-sized build. The Alps has it's own sound signature, and when compared to more expensive pots, you'll notice a nice difference. Many people swear by the TKD 2CP-2511 ($75), Noble AP25 ($25) and others; you really have to experiment to see where your preferences will lie.

There's another class of products for those overly concerned with taking good care of the input signal attenuation called stepped attenuators. Stepped attenuators can be set up in a series, shunt or ladder configuration through a network of fixed resistors and a rotary switch. There's an excellent comparison between these three configurations on DIYAudio by Arn Roatcap, Inc. A series attenuator (the most common) is the cheapest and has the fewest parts, it uses all resistors up to a given point as the voltage divider. The problem is the resistor noise is added up, the more that are in the way of the signal.

Series Attenuator - Image Copyright Arn Roatcap, Inc.

A ladder attenuator is in theory the best of the bunch, as it only uses two resistors at any one given time, this results in low resistor noise and good channel-to-channel signal level matching, it's the most expensive and largest of the bunch though.

Ladder Attenuator - Image Copyright Arn Roatcap, Inc.

Then there is shunt style, which is a voltage divider that is formed with one single value resistor, and one additional resistor selected by the position of the rotary switch. This style will vary input impedance so it requires one to consider the equipment it will be used with to avoid overloading the input signal source equipment.

Shunt Attenuator - Image Copyright Arn Roatcap, Inc.

Typically, a series stepped attenuator can range anywhere from a $25 cheapie on eBay to several hundred dollars. Ladder steppers require many more resistors and boards so the cost increases higher still. Some companies that make attenuators are Goldpoint, Elma, Shallco, Glassware Audio, DACT and Seiden. Elma, Goldpoint and DACT all seem to make use of similar rotary switch mechanics and are closely priced. Resistors may be full sized through-hole resistors or SMC. You can get them with the resistors presoldered or you can choose your own by figuring out the logarithmic scale you'd like to follow and how many dB of attenuation you want between each step. There are programs to help you decide this on tonnesoftware.com and Glassware among others. You don't really save much money going this route, but you can choose your own flavor of resistor this way and you'll likely be a little more proud of your completed build.



Here's an Elma stepped attenuator I have without resistors mounted:

Elma Stepped Attenuator

Here's one I populated with some Kiwame Carbon Film resistors:

Elma with Kiwames

It takes about two hours give or take to populate all these resistors and solder them in (in this case it was 48 resistors for 24 steps). You'll find that a stepped attenuator gives you a clearer and more detailed presentation than a low cost potentiometer. Is it worth the work or extra dollars involved? You'll have to be the judge of that ;)

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 modification posted below 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. 

Complete Restoration of a Dynaco PAS Pre Amp

This is a story of rags to riches; turning an abused Dynaco PAS 2 into a very modern PAS 3.

The Dynaco PAS 2, for those who are unaware, is a pre amp kit introduced by Dynaco in 1960 to compliment the Stereo 70 (ST70) that came the year before (1959) and other various other power amplifiers in the company's lineup. The Dynaco PAS has developed a cult following, and there are an immense amount of boards being produced for those interested in restoring such archaic technology :) CurcioAudio, Tubes4HiFi, AudioRegenesis, ClassicValve and others have created drop-in boards to restore the PAS, whether you want to maintain the same circuit or drop in something completely different.

I chose to use reproductions of the original PAS 3 boards and rather than recreate the rudimentary selenium rectifier with standard capacitors, I chose to make use of the high-tech ClassicValve PAS Regulated Power Supply.

ClassicValve Regulated Power Supply

The PAS is a design that was built to fit a budget, so a number of other things are recommended to be replaced. The 2 pole, 6 position switch is rather archaic compared to what is being produced today. A $10 switch would likely be an improvement. I opted to go with an Elma 2 pole, 6 position switch 04-1264 ($60 SRP) so that I wouldn't have to replace it down the line.

A Stock PAS Switch

The rear input and output board also leaves something to be desired. Many consider this to be one of the most drastic areas that Dynaco cut costs on the kit. Cheap Nickel plated RCAs on an old PCB board don't really inspire confidence.

Stock PAS IO Board

So... let's have a look at the splendid PAS 2 example that I was able to source:

Original PAS Back

Original PAS Back Detail

Original PAS Insides

Original PAS Outside and Bottom

The previous user left me with a lot more work that I would have had if I had a pristine PAS 2 / 3. There were a number of unusual mods, including plexiglass mounted RCAs, a 9 pin tube mounted in the middle, a non-descript transformer that was halfway mounted, some additional holes cut out of the back, and a powdercoated front panel that was rubbed with armor-all. No worries, one has to rise to the challenge.

The PAS was completely taken apart, a few parts were set aside, most were disposed of as they had been pillaged by mods. The chassis was sanded down and prepped for powder-coating.

Chassis Prep

The new boards were sourced and populated with my favorite parts, Kiwame and Takman resistors and Sonicraft caps. 

Old boards versus new boards

Curcio Audio has a number of nice tutorials on the PAS, including one to make the Phono board a bit more accurate. I performed those mods, along with bypassing the tone controls. I sourced a new input output board from Tubes4Hifi to connect to the Elma switch. I also picked up an Alps volume control (100K) and balance control, along with PCBs to make soldering a little easier.

Mostly done

The wiring was completed using Neotech PCOCC in teflon wiring, Kimber wiring and SPC in teflon. Russian teflon bypass caps were added to the main board (mounted to the front), and Russian PIO were used on the phono board. A power LED board was sourced from ClassicValve, and a new transformer was sourced from TriodeElectronics. After all was said and done, I sent off the PAS to Sal at Dynaco-ST70.com to look over all my wiring and fire it up. I wanted to have an expert give it the final eye and do the final wiring.

Here's the final build:

PAS guts from the right

PAS guts from the left

Inside Detail

Inside Detail

Bottom wiring

Outside with new gold front panel from ebay

Outside Photo 2

Rear, notice rear vent and Cardas output RCAs

Some final details were the Cardas RCAs added to use the holes made by the initial owner, a power supply vent to eliminate the ugly holes in the chassis, the bottom slots were covered using slightly larger than stock power receptacles from Radio Daze, and the front panel was replaced with a gold anodized version from Michael Nipomo on eBay (he typically only sells black and silver, but had this gold one as a one-off).

This little baby was an amazing amount of work, but it came out well and I'm really happy with it. Good luck to everyone out there restoring their PASs :)

Wondering where I got all my parts for this build? Check out a list of my favorite online vendors here.

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.