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.


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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.

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Review: Cardas Cross and Quadlink 5C interconnects

At the urging of a friend, I decided to replace my old Belden and Cardas Crosslink interconnects. For the phono preamplifier to linestage connection, I bought a used one meter set of Cardas Cross cables, which are considered mid-priced. Between the linestage and the amplifier, a new one meter set of Cardas Quadlink 5C cables were selected. The quality of construction leaves nothing to be desired as the rhodium RCA jacks fit like a glove and the cables are extremely flexible.

Why Cardas? They've been in the business for a long time and have a theory to cable construction that actually makes some sense (at least to me). Further details can be found on their website here.

Differences, compared to the older set of wire, are minor and not the type to be necessarily detected using an ABX box. To some, this means that the differences don't exist. But, psychological expectations aside, I still heard some changes. Trailing notes and minor 'inner' detail was bumped up a bit. The new Cardas cable also has a warmer, more organic presentation that manages not to sound sluggish. In general, my system took on a slightly more relaxed sound.

The big question that still remains is upgrading interconnects worth it? Realistically for the price, no. I've heard bigger diferences between cartridges, phono stages and whatnot. But like any other 'extreme' hobbies, nearly insignificant changes to one person make all the difference to another. In the end, my adventure into wire upgrades has given me a more enjoyable stereo. I couldn't ask for anything more except for a lower price!

System:
preamp: Threshold FET-10/HL
phono preamp: Threshold FET-10/PC
amplifier: Threshold S/500
analog: VPI HW19 MkIII - Rega RB300 with Incognito wiring - Dynavector 10x5
speakers: Magnepan 1.6/QR
speaker cable: Kimber 4PR/8PR Bi-wire with banana jacks
Interconnects: Cardas Crosslink and Belden


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Review: Kimber 4PR/8PR Bi-Wire Speaker Cables

I never thought I would be writing a review on speaker cables as differences between wire is a contentious issue. I've never been one afraid of disagreement, but in all honesty I've never heard any change I could honestly attribute to interconnects or speaker cables. And really, in my heart of hearts, I didn't want to hear differences in cable. The thought put fear into my wallet, not to mention my engineer orientated brain.

As part of recent system finalization, I've decided to replace the old Canare Quadlink speaker wire. I did this for several reasons. I had a fear that the spades could short against each other in the tight confines of the Threshold amplifier and I also wanted a bit longer reach than the six foot length afforded. My Magnepan 1.6/QR speakers can also be bi-wired, removing the metal straps between the high and low inputs. With that in mind, I ordered an eight foot pair of Kimber Kable 4PR/8PR Bi-Wire speaker cables terminated with bananas.

Upon arrival, I removed them from the supplied packaging and was impressed by the construction quality. Though the 4PR / 8PR cables are Kimber's most budget cable, the termination and braiding job was well done. The 4PR consists of four brown and four black conductors braided together to make a 14awg cable. The 8PR is built the same way with eight cables per leg, producing a 10awg cable. I removed the Canare wire from the system and also removed the hi/lo straps from the Magnepan speakers. Installation of the Kimber cable was a breeze as I used the 4PR set for the tweeter and the 8PR set for the woofer panel. I then settled down to give my system a quick listen.

Initial impressions were certainly interesting and not at all what I expected. At this price point I expected no change at all, but was instead greeted with a brighter but edgier treble. Bass extension also seemed deeper, perhaps the 10awg cable helping with the high amount of current I use to drive the Magnepan speakers. Deciding some more break-in may be required, I let the system play in the background in the evenings while I worked out.

A few days later, I had time for some further listening, which revealed the Kimber 4PR/8PR to be one smooth cable. There is a warm coherency from top-to-bottom and the bright edginess of the treble disappeared. The speaker drivers, a slightly strange term to use with Magnepans, seemed to now play as-one. The lowest bass, never a strong point on the Magnepans, was better too. Though the differences were not staggering compared to the old Canare, the changes were still audible.

So there you go - my prejudices once again have been shattered. I never thought solid-state amplification could cut it and now I'm hearing differences in cables. I think part of the reason for this change comes from finally having a better front end, revealing speakers and a certain amount of stability. Before, I was always trying new amplification (often DIY), speakers and moving to different rooms. Such circumstances clouded my ability to discern subtle changes as the amount of given variables were too high. Now once I get the chance, I will be willing to spend the dough on some even more expensive cables. Somewhere I hear my wallet screaming.

System:
preamp: Threshold FET-10/HL
phono preamp: Threshold FET-10/PC
amplifier: Threshold S/500
analog: VPI HW19 MkIII - Rega RB300 with Incognito wiring - Dynavector 10x5
speakers: Magnepan 1.6/QR
speaker cable: Canare Quad
Interconnects: Cardas Crosslink and Belden


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A visit with Magnepan 1.6 speakers

The Magnepan 1.6/QR is a 2-way Quasi-Ribbon Planar speaker. My previous experience with planar speakers had been limited to a pair of Quad ESL-63s and a set of Magnepan SMGs. The Quad ESL-63 has always been one of my holy grail speakers but I was worried about children, pets and my own clumsiness around a high voltage electrostatic panel. The Magnepan seemed like a good compromise as I had a high current amplifier to run them with.

Upon arrival of the speakers, I had my fellow audiophile come over and help me handle the long 80lb box down my basement steps. The 1.6s were soon unboxed, unwrapped and mounted on the stock speaker stands. They certainly are imposing speakers, standing just a bit shorter than me. I shuffled the Maggies about a bit, pulling them some four feet away from the back wall and some three feet from the sides. I threw on a quick CD to make sure everything was working and was happy to hear music coming out of both channels. My friend suggested we try a known LP, so I threw on a copy of Steely Dan's 'Aja' and settled back into the sofa.

My ears were met with a very aggressive bright forward sound but as my ears started getting used to the change in sound I found that the soundstaging and details were simply amazing. Cymbals and voices were lifelike, making my old box speakers sound muddy in comparison. Bass response was different and lighter than the UREIs - which is to be expected since those big speakers have two 15" Eminence drivers. However the Magnepan's bass was extremely fast and articulate.

We then threw on an original copy of the Beatles 'Rubber Soul' and were amazed by the vocal delivery and realism. But there was still a bright spotlight on the upper treble that was annoying. My friend suggested a cartridge loading change. My Dynavector 10x5 was currently loaded at 47k which can cause a rise in the treble. So I took off the top of the Threshold FET-10/PC and via the dipswitch changed the loading to 1K.

The change was certainly for the better as the next record showed - Willie Nelson's 'Stardust' was sublime as was Van Morrison's 'Saint Dominic's Preview'. Further listening on a variety of records revealed a speaker that has a tremendous amount of detail and the ability to throw a large soundstage. Instruments and voices hung in their own space and the music flows as an organic whole. With many standard dynamic speakers you can have (let's say) a gentle ticking of a stick that gets washed out by the crescendo of a guitar. However with the Magnepans the picture never fell apart like that, keeping the different intruments and vocalists distinct from each other. This neat trick allowed me to 'see' into the recording further than before.

Weaknesses? Lower bass doesn't growl like my big UREIs. Bass isn't exactly deficient, but it doesn't plummet the depths like big box speakers do. There is also less 'slam' and dynamics to drums as the excursion of the Magnepan can't match that of a set of big woofers. The Magnepans are also a little cruel with poor recordings, but the music was still enjoyable. Any speaker is a set of trade-offs but in their price range the Magnepans are certainly worth checking out. Highly recommended!

Stay tuned as I'll be playing with stands and perhaps a crossover upgrade in the future.


System:
preamp: Threshold FET-10/HL
phono preamp: Threshold FET-10/PC
amplifier: Threshold S/500
analog: VPI HW19 MkIII - Rega RB300 with Incognito wiring - Dynavector 10x5
speakers: UREI 813A loudspeakers
speaker cable: Canare Quad
Interconnects: Cardas Crosslink and Belden


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