Thursday, May 20, 2010

Upgrading the Dynaco PAT-4 preamplifier

Ever since I've been into audio, the Dynaco PAT-4 has gotten a fairly deserved bad rap. I first owned one of these vintage solid-state preamplifiers back in 1990 and the sound, even with a tube amplifier, was certainly nothing to remember - harsh, flat and very unpleasant with any speaker with a working tweeter. Eventually the whole thing was gutted and used as an experimental op-amp platform!

Flash forward twenty years and I'm stuck with a large Ikea Expedit bookcase with 13 by 13 inch slots. Though perfect for holding hundreds of LPs, finding quality small gear that can fit inside has always been a challenge. The Dynaco line of preamplifers, tuners and integrateds slide right into the slots perfectly, which is one of the reasons I keep coming back to these venerable components. Of course another reason is the easy to get manuals and parts which makes modifications and repairs easy on the wallet.

The Dynaco PAT-4 was designed by the late Ed Laurent who had a hand in many of Dynaco's more famous products. An examination of the schematic reveals a simple single power supply, two transistors for each channel of the phono section and two transistors for each channel of the line section. Negative feedback is used to control overall gain and lower output impedance as there is no buffer stage to drive the output. This circuitry actually reminds me of the Dynaco PAS and probably came directly out of the transistor manufacturer's specification sheet.

I began to wonder if this old preamplifier could be made good with a few part upgrades, so I decided to tackle upgrading a Dynaco PAT-4 using the AVA Audio 1986 Audio Basics (starting on page 29) as my basic guideline. I soon found a nice looking PAT on Ebay for an absurdly low price and upon arrival was greeted by a fairly mint specimen. I hooked it up to my McIntosh 250 and was not impressed by the results. The PAT-4 was as harsh and grainy as I remembered it, plus one of the phono channels was weaker than the other. My old PAS certainly bettered it on all levels except the PAT-4 did have a slightly better rhythmic bass. Could I gain any sonic improvements by replacing the ancient capacitors and bypassing the touchy tone controls? Let's find out.

Cracking open the PAT-4 revealed a bevy of leaking electrolytic coupling capacitors and even two connections that were missed by the solder iron of the original kit builder. Using some cut up white address labels, I marked the wire connections to the PCB boards and soon had the whole PCB assembly removed. This is also a good time to replace the four small electrolytic capacitors located near the front input jack and back tape outs with wire.

With the tone control bypass, I also figured out a clever way of removing one of the large coupling capacitors from the signal path while keeping the emitter bias dc voltage out of the signal output. See image below for the tone control bypass instructions and the capacitor upgrade locations.

As far as parts quality, this is a budget rebuild so I shied away from the ultra-expensive tweak parts. Instead, standard Digikey parts were used except for the four 10uF Polyester signal and Nichicon Muse power supply capacitors I found cheaper on Ebay. Replacing the components on the PCB boards is easier if the metal brackets are gently bent to a 90 degree angle.

The power supply was next. I originally envisioned a LM317 regulator, but feared the original transformer could not handle the extra current demands. Dynaco built everything to a price point and I wasn't sure if the transformer would overheat. Since this is a budget build anyways, I decided on just improving the basic unregulated supply with the use of slightly more capacitance and better parts. Three 1000uF/63V Nichicon Muse capacitors mounted on a terminal strip replaced the old can cap. A more intrepid builder could remove the original transformer, use an outboard SMPS supply that feeds an onboard regulator using a LM317 (or whatever) .

After checking supply voltages and DC offset, I hooked the Dynaco PAT-4 as a phono only preamp into my main Threshold/Magnepan system. The sound wasn't half-bad. A little lacking in 'color' and drama, the signal was clean and certainly listenable. It vaguely reminded me of an old Adcom 555 preamplifier, but my memory of that unit is fairly sketchy, rendering any comparison fruitless. Compared to the more expensive Threshold FET-10/PC, the refurbished PAT-4 was definitely second tier material with a slight sterility to the sound.

On the upstairs second system, the Dynaco PAS-3X was taken out and the PAT-4 installed. Paired with the McIntosh MC-250, this could have been a fairly common solid-state system back in 1969! The sound was certainly different than the previous preamplifier as this classic all solid-state combination was faster and had deeper bass. Some fairly minor upper-midrange/treble glare still existed with the use of PAT-4, but has been greatly reduced by the film capacitors and newer electrolytics. The new sound is now quite modern and the phono stage is also extremely quiet without any channel imbalance.

The refurbished Dynaco PAT-4 is now a nice little budget preamplifier and would be an inexpensive project for anyone handy with a soldering iron. Your results may vary depending on the new parts used and the overall condition of the stock unit.

Second System:
Dynaco PAS-3X modified
Dynaco AF-6 tuner
McIntosh MC250 amplifier
Panasonic linear tracking turntable
Pioneer DVD-V7400 cd player
various budget cables

(05/23/10) Update: Component break in? I don't know, but a few more hours on this preamplifier and it has really smoothed out. Most of the glare is gone and I'm quite pleased with the end results. For a budget system, I can really live with the refurbished Dynaco PAT-4. With new parts and the tone control bypass, it is a screaming deal for the little time and money put into it. The phono stage is dead quiet and the linestage is very quick and dynamic. Music is tons of fun to listen to with good detail, deep bass and snappy transients. Not bad for something that's normally considered a vintage piece best forgotten.


Tuesday, May 11, 2010

Upgrading the Dynaco SCA-80 integrated amplifier

Some things in life are always worth doing, while others require a furrowed brow. Upgrading a Dynaco SCA-80Q amplifier doesn't exactly define common sense as the design is antiquated at best. However my Ikea record/bookshelf has narrow 13" slots and I already had a matching Dynaco AF-6 tuner that fits nicely. A $25 Ebay win soon had a dusty and decrepit '80Q' in my hands.

Featuring the Dynaco PAT-4 preamplifier and ST-80 amplifier all in one chassis, the 'Q' version added a primitive 'quadraphonic' sound with the use of a single 10 ohm resistors. Looking at the schematic of the SCA-80Q reveals a single common power supply, no regulation, a simple (almost tube-like) topology, and a plethora of electrolytic coupling capacitors. Surely much of the associated harshness must come from the aged parts and poor power supply design.

An article from Van Alstine pointed the way to bypassing the tone controls and a few dollars at Digikey and Ebay soon had some parts speeding my way.

1) Bypass the tone controls by snipping out the tone control wires that lead to numbers 13-18 on the preamp PCBs. On the PCBs, tie leads 13-15 together. See page 29 of the 1986 Audio Basics newletter.

2) On the preamp PCBs, replace the six 50uF/25V Elna electrolytics with 10uF/100V film types. I used some low budget metallized units. I also replace every other low voltage electrolytic as these can be troublesome. Make sure to also replace the two grey electrolytics coming off the PCB boards, located behind the volume control, as these are from the phono stage output. This whole capacitor swapping process is easier if the PCBs are removed from the unit. Of course this requires marking the location of the old wires and some deft use of the soldering iron.

3) Replace 5000/80V power supply capacitor with a 6800uF unit. Replace the four 400 ohm / 7W units on the back power supply PCB with four 390 / 10W units. On the stock unit, this board is known to start on fire, so I kept the new resistors directly off of it.

4) Replace the two side mounted 5000F/80V speaker coupling capacitors. I used some nice modern 4700uF Panasonic units bypassed with some 4.7uF/250V caps from my junk box.

5) The white wire wrapped around the speaker coupling capacitors are actually small valued chokes. I bought a smaller packaged pair from an Ebay seller who specializes in Dynaco parts and mounted them on some terminal strips.

6) I decided to regulate the preamp section by using a simple 7824 regulator. Electronics 123 has a nice like PCB to accomplish this but the 7824 still needs to be bought from Digikey.  This required the changing of the original 4.7k dropper resistor to a 400 ohm / 10W unit. I also changed the phono section power supply to come from the regulated supply and I used a 1.2K / 2W dropping resistor.

Due to laziness and the number of wires to desolder from the board, I did not tackle the amplifier PCBs yet. There are still some electrolytics that need replacing and I have an idea of improving the negative feedback as the output caps and chokes are not included in the loop.

So how does the amplifier sound? Quite modern now. With my KEF Q60s, much of the harshness is gone and only some faint residual 'solid-state' upper midrange hardness can be heard. I was surprised how dynamic the amplifier sounds now as rhythm gels better than my trusty Dynaco PAS-3X preamplifier and McIntosh 250 amplifier. The tube / solid state combination does sound smoother and warmer, but lacks the impact and speed of the refurbished Dynaco SCA-80Q.

Note: for a more detailed upgrade, check out the latter posts on the Dynaco PAT-4 and ST-80 upgrades.