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The PAT-4 Pages

Front View of thePAT-4 with a blue light kit installed

About the Dynaco PAT-4 Preamp

The PAT-4 was Dynaco's companion solid-state preamp for the Stereo 120. The PAT-4 pages have lots of information about it, starting with schematics. As you work your way through, you'll see:

Dynaco PAT-4 Preamp Schematics, Assembly and User Manual

The following links are a scan of a nice, clean copy of the PAT-4 assembly and user manual. The files are both kind of big because the scans are high res. You can zoom in tight to see what-ever you need, since they were scanned at 300 dpi. They even include the large fold-out diagram.
PAT-4 Assembly Manual, Part 1
PAT-4 Assembly Manual, Part 2

Dynaco PAT-4 Phono Preamp Schematic with DC voltages

If your phono preamp is not working, a good place to start is with a check of the DC voltages. That was never listed in the manual, so I've prepared this figure with the no signal DC voltages on the nodes in the phono preamp.

phono preamp schematic with DC bias voltages annotated

PAT-4 Preamp Upgrades

The number of PAT-4 Preamp upgrades has grown through the years, so I felt it was time to reorganize this page. Here's a short description of all of the upgrades, with a link to more information.

Dynaco PAT-4 Preamp Tone Controls

Although the tone controls have been undeservedly disparaged through the years, the unique Dynaco design makes the center position, +/-5% of the rotation absolutely flat. With proper adjustment, this takes the tone controls out of the circuit when in the 12 o'clock position. The next two items give information about the bass and treble controls, and how to assure that flatness.

The PAT-4 Bass Control

The PAT-4 Bass Control potentiometer is unique in that Dynaco way. The document here shows how to make sure your control is centered for flat bass response, and how to clean the control. It also provides data about the rather unusual characteristics of the control. PAT-4 Bass Control Pot gives you everything you need to know.

The PAT-4 Treble Control

The PAT-4 Treble Control potentiometer is also unique in that Dynaco way. The document here details a method that assures flat treble response when the control is centered. It also shows how to clean the control and provides data about the control's unusual characteristics. PAT-4 Treble Control Pot gives you everything you need to know about the treble pot.

Is Your Bass Control Scratchy?

You say you've already cleaned your pots with contact cleaner, but the bass control is still kind of scratchy. If so, it's very likely that C16 has become leaky. Its probably best to just replace C16, but theres an interesting leakage test you can perform:

  1. Work safely, disconnecting the PAT-4 from power before working on it.
  2. Desolder the negative end of C16.
  3. Connect the desoldered negative end of C16 to the positive lead of a DC micro-ammeter. Connect the negative lead of the meter to ground.
  4. Remaining clear of the exposed 120 Volts, plug the PAT-4 in and power it up.
  5. Observe the DC micro-ammeter. As the capacitor charges at first, there will be significant charging current. After a few seconds, it should subside to zero.
  6. Note that 0.02 mA (20 uA) is definitely not zero! A typical bad capacitor will have that much leakage. A good capacitor should have much less than 1 uA of leakage current.

C16 is originally 50 F, but 47 F is an equally fine replacement. It should have a working voltage rating of greater than or equal to 25 Volts.

Why does C16 become leaky? It sits right above R20, a 620 Ohm, 1 Watt resistor that dissipates about 640 mW. In most of examples Ive seen, C16 actually touches R20. The heat is probably the cause of the failure. Replacement caps typically have a bit smaller diameter, putting some space between themselves and R20. This should enhance the life of the new cap.

Do you need a New Treble or Bass Control?

I've recently come across a source of New Old Stock (NOS) parts for the PAT-4. The bass and treble controls have special tapers as described above. I thought that they were not available anymore, but Vintag Electron has an Ebay store that, at least for now, is carrying some PAT-4 parts (bass and treble pots, rocker switches) that I haven't found anywhere else. I haven't checked any of the parts myself, but he has a great feedback rating, so his stuff is probably the real deal. Just follow the link, and put PAT-4 into his "Store Search" box.

Bypass your Tone Controls with this Switchable Tone Control Disable Kit

You can guarantee that your tone controls are flat, or just have an easy way to compare a flat setting to a tone control shaped setting. You can add a tone control switch that replaces the HI FILTER switch that almost no one ever uses. I've almost finished working on a kit that adds a tone control disable switch to the PAT-4. You can optionally drop the gain of the flat gain stage a bit also...that can increase the SNR.

Switchable Tone Control Disable Installation Manual.

Buy the Switchable Tone Control Disable

The PAT-4 Volume Control

The PAT-4 volume control pot is listed as 250K. I measured one in one of my PAT-4's. One track measured 194K. The other track measured 185K. Either these were a bit anomalous, or the real resistance of the pots was closer to 200K than to 250K. The loudness taps were both up 41K Ohms from the bottom of the pot. At rotational extremes, the wiper comes within about 40 Ohms of the ends of the pot.

Replacing the PAT4 Volume Control

These kind of volume control pots, with the loudness tap, are becoming somewhat scarce. One reasonable replacement actually comes from Radio Shack. It's their part number 2711732, a 100K Stereo Audio Pot. There are some differences:

Comparing the Pots
Parameter Dynaco Radio Shack Comment
Total R section 1 185K ohms 111K
Total R section 2 194K 100K
R tap to gnd end 1 41K 13K2
R tap to gnd end 2 41K 13K9
Shaft diameter 0.250 inches 0.234 Pretty close
Shaft type D shaped Round Works fine
Bushing diameter 0.375 inches 0.266 Radio shack needs washer

However, as we used to say in the music business when tuning, "It's close enough for jazz". You may have to bend the locator tab off the Radio Shack part.

Electronically Regulated Power Supply Kit for the PAT-4

The original PAT-4 power supply was a beautiful compromise for a time and a world where transistors cost more than capacitors. The output voltages were fairly quiet, though completely unregulated. They would drift around with the line voltage, with the temperature of the transistors in the phono preamp and high level stages, when the refrigerator compressor kicked get the picture! At high frequencies, its output impedance was low, but at low frequencies, the impedance was 1000's of Ohms. Power supplies, ideally, should have low impedance and low noise everywhere!

Now, at some point, people addressed the PAT-4 shortcomings by "throwing a mess-o-microfarads" at the problem. That made the power supply better, but still left rather high Z-out at low frequencies. Further, it still wasn't regulated. Low or high line conditions were still directly reflected in the output voltage. The effects of other appliances in the house turning on still came through the PAT-4 power supply.

The new PAT-4 Power Supply Kit adds electronic regulation to the power supply equation, along with enough microfarads to keep everything quiet. It's easy to build and install. Just remove the silver can cap, and mount the new power supply using the same mounting holes. It maintains a slow start so there are no clicks and pops even if you turn it on while connected to an energized power amplifier. All the output impedances are less than 1 Ohm, and the noise is almost too low to measure.

We also have re-designed and re-routed the transformer secondary wiring to decrease the loop area, which minimizes external fields caused by power supply ripple. For the rest of the details, please take a look at the PAT4PWR ordering information and Assembly Manual.

A Really Nice Example of the Installation

Gerhard, from New York, sent along these pictures of his PAT-4, with the PAT4PWR kit installed. Note the tight twist on the transformer wires which greatly diminishes external hum fields. Note also how Gerhard has dressed the wires and collected them with cable ties. The result is a PAT-4 that looks great and sounds great!

PAT-4 with PAT4PWR, side view

PAT-4 with PAT4PWR, back view

PAT-4 with PAT4PWR, top view

Decrease the Line Stage Distortion by a Factor of 10

All the sound in the PAT-4 passes through the high level stage. If only there were a simple way to dramatically decrease the distortion, you could improve the whole preamplifier.

There is a simple way to drop your distortion by 10X

You can install our DRD4 Distortion Reducing kit, and drop the high level stage distortion by a factor of 10.

Here are some specifics: With 1 Volt RMS into a 100 KOhm load, the PAT-4 has about 0.02% distortion. That's pretty good, but it could be 10 times better! With the DRD4 kit you just change one of the stock transistors in both left and right channel high level stages, and you will drop the distortion from 0.02% to 0.002%. We supply the specially selected darlington transistors, heat-sinks and thermal compound. Of course, you get the usual easy-to-follow installation manual.

Distortion Reducer Installation Manual.

Buy the Distortion Reducer.

Adding More High Level Inputs

The PAT-4 has a six position input selector switch, 3 low-level inputs and 3 high-level inputs. Very few people use the Special and Tape-Head inputs. If you need more high-level inputs, you can convert them to accept high level inputs. The step-by-step instructions that follow were posted by Dr. Tinear to the AudioKarma forum. (Note: I haven't tried the modification myself). It is posted here with Dr. Tinear's kind permission.

Here are Dr. Tinear's step-by-step instructions on converting the PAT-4's Special and Tape Head inputs to line-level inputs. You'll need the PAT-4 Preamp pictorial wiring diagram to identify the switches and the lug numbers on each. The phenolic wafers on the selector switch are fragile, so be very careful when you shift wires or do soldering on that switch.

1. Remove the following wires at the rear panel:
- Short Lug 2-3 to Short Lug 4-5
- Short Lug 13-14 to Short Lug 15-16

2. Add the following wires at the rear panel:
- Short Lug 4-5 to Short Lug 6-7
- Short Lug 15-16 to Short Lug 17-18

3. Remove the following wires between the front SS wafer and the front PC-16:
- SS lug 4 to PC-16 eyelet 6
- SS lug 3 to PC-16 eyelet 7
- SS lug 2R to PC-16 eyelet 8
- SS lug 5F to PC-16 eyelet 1
- SS lug 2F to PC-16 eyelet 5

4. Remove the following wires between the rear SS wafer and the rear PC-16:
- SS lug 4 to PC-16 eyelet 6
- SS lug 3 to PC-16 eyelet 7
- SS lug 2R to PC-16 eyelet 8
- SS lug 5F to PC-18 eyelet 1
- SS lug 2F to PC-16 eyelet 5

5. Disconnect the wire from front SS lug 8 and connect it to front PC-16 eyelet 1.

6. Disconnect the wire from rear SS lug 8 and connect it to rear PC-16 eyelet 1.

7. Add the following wires from the front SS wafer to the front PC-16:
- SS lug 8 to eyelet 5 (do not solder at the SS lug)
- SS lug 8 (solder 2 wires here) to eyelet 6

8. Add the following wires from the rear SS wafer to the rear PC-16:
- SS lug 8 to eyelet 5 (do not solder at the SS lug)
- SS lug 8 (solder 2 wires here) to eyelet 6

9. Add the following wires at the selector switch:
- Front SS lug 5F to front SS lug 2F
- Rear SS lug 5F to rear SS lug 2F

10. (Optional) Remove the 7500 ohm resistor between AS lug 2 and AS lug 5. Replace it with a short length of wire, making sure that the wire does not contact any of the other switch lugs. This change gives you full A+B mono rather than 6 dB of stereo separation when you set both the A and B switches to Mono.

A Complete Phono Preamp Replacement and Upgrade for your PAT-4 Preamp

assembled phono preamp for PAT-4

The PAT4LP kit was the original way to upgrade your phono section. If you like keeping an original look inside your PAT-4, that's probably the way to go. But, if you don't mind drilling two holes, you can add a brand new phono preamp circuit board and pick up a lot of advantages that come with the freedom in a new PCB design.

Here's a picture of the phono preamp, installed in a PAT-4, right near the selector switch

Preamp Installed in a PAT-4

Order the PAT-4 Phono Preamp Replacement and Upgrade Kit

How Old is Your PAT-4?

Factory built units have date stamps inside the chassis, between the circuit boards and the back panel, on the floor of the chassis. One of mine says, "SEP 78". I'll be so bold as to generalize this to say all factory assembled PAT-4's have a 3-letter month code and a two-digit year code identifying their date of manufacture.

Experiments in Progress

Update to this Experiment -- While some of the things here are certainly valid, the biggest single thing that you can do to reduce the hum is to install the PAT4PWR board. It has lower hum at the output. More importantly, it handles the AC rectifier voltage in a much better fashion, dramatically decreasing the loop area of the rectified currents. This makes a HUGE decrease in the reported hum levels in a PAT-4.

You can also reduce hum with the PAT4X kit. It replaces the original transformer with a low-noise, low external field, toroidal transformer. But, you can also spend less money, and get some worthwhile on!

I'm trying to reduce the noise, both hiss and hum, in a PAT-4. The first picture shows the original circuit board. The second picture, a bit of the disassembly process. Note the tags on the wires to help it all go back together!

unmodified PAT-4 circuit board

unmodified PAT-4 circuit board

disassembling a PAT-4 to remove a circuit board

disassembling a PAT-4, tagging and removing the wires

We're back together again. I changed out a noisy 390 Ohm resistor in the emitter of the rigth channel tone control circuit, and dropped the broadband and 1/f noise by about 8 dB! That was one noisy resistor. The observation came about because both channels were not equally quiet.

If you have some extra hiss in your high level stage, you might get equally lucky with a "rifle-shot", changing out that 390 Ohm resistor. Still, to get it out, you have to disassemble your PAT-4 and pull the PC boards. Heck, while you're at it, why not replace all the resistors and electrolytic caps in the high level stage? You can do that with the P4LSRC kit.

The next experiment showed that with the volume control at minimum, the twisted pairs from the balance pots to the tone control amplifier inputs were kind of sensitive. With the cover off, and the system hooked up, touching your fingers to the insulated wire of the twisted pair adds audible buzz. I replaced the twisted pairs by legitimate shielded cables with a foil shield and a drain wire. You can now touch the outside of the shielded cable without picking up added buzz. This mod cut the hum reading by about 1/2 (I did this experiment at home, where my instrumentation is a bit crude). Hum usually comes from many contributing sources, all of which must be eliminated. This mod eliminated some of the hum, and will be maintained. But there's still more hum to find and remove. The pictures below show the shielded cable modification.

shielded cable from balance control to amp board, view from front

we changed twisted pairs to shielded cables from balance control to amplifier boards (viewed from the front)

shielded cable from balance control to amp board, view from back

back view of the mod, changing twisted pairs to shielded cables from balance control to amplifier boards

This document describes the process of preparing the ends of the shielded cable. You may also find that it's worthwhile to use shielded cables from the input jacks to the selector switch. The shield should only be connected at the input jack end.

About the Goal-is it possible?

As our first goal, we're trying to get a PAT-4 with the volume control at minimum, running into an updatemydynaco stereo 120 (voltage gain of 21), so free of hum that you can put your ear up to the speaker, and hear just hiss, and no hum. Let's see what this might really mean. 1 Watt into a typical speaker produces an SPL of 90 dB at 1 meter. If we put our ear about 0.1 meter from the speaker, then the SPL is about 20 dB more, or 110 dB. To get to the threshold of hearing, we need to drop the SPL to 0 dB. That says that the hum power in the output of the power amp has to be 10 pico-watts. That's 9 microvolts at the output of the amplifier, or 9/21=0.42 uV at the output of the preamp. Since the preamp also has a gain of 10, that means we'd need to pick up less than 0.042 uV of hum at the input of the preamp. Wow!

Now perhaps I'm a factor of 10 too generous with my power and ear sensitivity assumptions, so maybe we could really stand 1.37 uV of hum at the preamp's output. Measurements of preamp hum to date show around -100 dBv , or 10 uV. We're really shooting for -120 round numbers, 1 uV of hum at the preamp's output. That won't be easy with the PAT-4's gain of 10 stage preceded by a volume control! We hope to report on today's experiments shortly.

It's Possible, But is it practical?

My most recent experiment drove the noise (hum) floor way down, with any harmonic of hum driven to the white noise floor or below. I fed DC from an external power supply to the first capacitor after the rectifiers. I did not feed AC power to the internal power supply transformer. These steps are necessary, but are not sufficient.

The PAT-4 still picks up stray external magnetic hum fields. Next, I placed a 0.062 inch thick sheet of very high permeability mu-metal, Co-netic AA, on top of the PAT-4. The sheet was a bit wider than the footprint of the PAT-4. As Emeril would say, "BAM"...the harmonics were gone. That mu-metal is too expensive for most applications, so my next experiments are to see how much less mu-metal we can get by with, and to verify the hum killing experiment in a real listening test.

Library of Grounding Problems

The experiments on this page started me thinking about and researching ground, hum and noise problems. When I think, I's a way to sort things out, and preserve them for my future self. You may find it interesting also. Download the Library of Grounding Problems.

Trouble-shooting Balance Problems

Here's a good way to figure out beyond a doubt if the balance problem is in your PAT-4, or the associated equipment. Trouble Shooting Balance Problems.

A Simple PAT-4 Improvement

It seems no one really likes the PAT-4's tone controls. In addition, the gain stage typically has more gain than it needs, resulting in more hum and noise. This is especially so if you've updated your Stereo 120 with the Updatemydynaco amplifier modules, as they are 3 dB more sensitive than the original modules. If you're interested, you can download PAT-4 Improvements.

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