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Dynaco PAT5 vs BiFet Phono Circuit

shelly_d

shelly_d

Not An Audiophool
Hi all:

I have a question about the Dynaco PAT5. The one I own I've had since new and it is NOT the BiFet. The only schematic I can find online is for the BiFet version. That had a switch on it to provide an additional 6dB gain on the phono preamp to allow a moving coil cartridge with it. I've go a couple of questions about this.

On the BiFet version, the emitter resistor of Q1 on the phono board is 470 ohms. Operating the switch adds a 412 ohm resistor in parallel to that.

I do not have access to a schematic for my version. I am reluctant to try to dig into my factory assembled unit before actually attempting what I'm considering. I have a few questions, perhaps not all answerable by the same person.

1. Does the non BiFet and BiFet version share the same phono circuit, especially in respect to that 470 ohm resistor?

2. If anyone has the BiFet version, does the phono section sound the same to you in either position or does the added gain change the sonic character of the preamp?

3. Has anyone found the moving coil position inadequate for the output of a moving coil cart either sonically or with respect to the gain? If sonically, did you also find the same issues with the phono preamp set for moving magnet carts?

Thank you all for any help you can provide.


Shelly_D
 
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The secret to the bifet is the has a faster slew rate . There are numeroius IC that sound much better than the original .You just have to buy one in the correct package and replace the one in your preamp the orinal one is pretty lame as i remember .Newer IC's are lower noise also .
 
The BiFET op amp -- a National Semiconductor LF357, replacing the LM301 in the non-BiFET PAT-5 -- is in the line stage, not the phono preamp stage. The PAT-5 phono preamp is a discrete-transistor design. If memory serves, the two versions of the PAT-5 used the same phono circuit, with the high-gain switch as the only difference between the phono sections in the two preamps.

I've never owned a PAT-5 BiFET, so I can't offer any comments on the phono preamp's sound quality or any differences that might be attributed to the High Gain switch.
 
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battradio said:
The secret to the bifet is the has a faster slew rate . There are numeroius IC that sound much better than the original .You just have to buy one in the correct package and replace the one in your preamp the orinal one is pretty lame as i remember .Newer IC's are lower noise also .
Click to expand...

Dr Tinear said:
The BiFET op amp -- a National Semiconductor LF357, replacing the LM301 in the non-BiFET PAT-5 -- is in the line stage, not the phono preamp stage. The PAT-5 phono preamp is a discrete-transistor design. If memory serves, the two versions of the PAT-5 used the same phono circuit, with the high-gain switch as the only difference between the phono sections in the two preamps.

I've never owned a PAT-5 BiFET, so I can't offer any comments on the phono preamp's sound quality or any differences that might be attributed to the High Gain switch.
Click to expand...

Thanks guys. I did know about the difference between the two line stage Op-Amps. This is about getting to use a Moving Coil cartridge with the equipment I currently have. I like, and always have liked, the preamp. Maybe my ears have turned to mush after 61 years of using them but that's where I'm at for now.

Thanks

Shelly_D
 
battradio said:
Matching transformer would be the easest way to for the moving coil cartrige .

the switch changed the feedback to get more gain , with a different ic you could get more gain by changing one resistor on each channel .
http://elektrotanya.com/dynaco_pat5_sch.pdf/download.html

A LF411CH would be a good choice for the ic ,double the value of R21 for more gain
Click to expand...

To clarify what I said in my earlier post, there's no op amp in the phono preamp section of a PAT-5 or a PAT-5 BiFET. The phono preamp uses discrete transistors. The High Gain switch changes the feedback to the first transistor in the circuit to increase the overall stage gain. Changing R21 will alter the gain of the line stage, affecting the preamp's overall gain on all inputs.

That said, I agree that a step-up transformer is probably the best approach to interfacing an MC cartridge to a PAT-5. The stock phono preamp has only 37 dB gain at 1 kHz, and installing the High Gain resistor mod, with or without the switch, only boosts it to 43 dB. That's not enough for most LOMC cartridges.
 
Dr Tinear said:
...
That said, I agree that a step-up transformer is probably the best approach to interfacing an MC cartridge to a PAT-5. The stock phono preamp has only 37 dB gain at 1 kHz, and installing the High Gain resistor mod, with or without the switch, only boosts it to 43 dB. That's not enough for most LOMC cartridges.
Click to expand...

Thank you sir. This is the kind of information that I needed to make an informed decision.

Any thoughts on how much gain is needed for a low output moving coil?

Thanks

Shelly_D
 
mhardy6647 said:
ballpark... 10 uV (1 x 10E-5V) to 1V = 5 orders of magnitude or 50 dB.
Click to expand...

OK: I've got 5 orders of magnitude, voltage gain of 100,000 times. However, I thought that the gain in dB for voltage gain is:

20 x log (V out / V in).

If that's right, then 20 x log (100,000) = 100 dB.

Am I wrong about something here?

If that's correct, then the 43 dB of the high gain position is very clearly not enough.

Thanks

Shelly_D
 
battradio said:
With transformers it a mater of impeadance not gain persay .
What cartrige are you looking at .
http://www.bobsdevices.com/whyshould.htm
http://www.tnt-audio.com/accessories/mc_3xfr_e.html
http://www.bobsdevices.com/cinemag.htm
Click to expand...

Right now I'm just looking at the possibilities. What's been holding me back is the cost, the cartridge plus the step up transformer has been just way too much money for me. Been out of work way too long to consider that much for stereo gear, especially with my current cart. when i ran across the low, high gain circuit I thought it opened up possibilities. Clearly it does not, not with the output voltages mhardy6647 has noted. Clearly the Dynaco, even in high gain mode, is short almost 60 dB (If my calculations are right) of gain.

Shelly_D
 
shelly_d said:
OK: I've got 5 orders of magnitude, voltage gain of 100,000 times. However, I thought that the gain in dB for voltage gain is:

20 x log (V out / V in).

If that's right, then 20 x log (100,000) = 100 dB.

Am I wrong about something here?

If that's correct, then the 43 dB of the high gain position is very clearly not enough.

Thanks

Shelly_D
Click to expand...

10 uV seems low for an MC cartridge output voltage. The ones I looked at online -- the Denon DL-103, a Dynavector and a Benz -- came in at 0.3, 0.3 and 0.5 mV respectively. That's roughly a factor of 10 lower than a typical MM or IM cartridge. A phono preamp with a gain of 60 dB or so at 1 kHz would do the job, or you could use a step-up transformer with a turns ratio of 10 (=20 dB voltage gain) ahead of the PAT-5's phono preamp. Remember that your PAT-5 has an active line stage with 20 dB voltage gain, so the phono preamp doesn't have to provide all of the system gain ahead of the power amp.
 
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Dr Tinear said:
10 uV seems low for an MC cartridge output voltage. The ones I looked at online -- the Denon DL-103, a Dynavector and a Benz -- came in at 0.3, 0.3 and 0.5 mV respectively. That's roughly a factor of 10 lower than a typical MM or IM cartridge. A phono preamp with a gain of 60 dB or so at 1 kHz would do the job, or you could use a step-up transformer with a turns ratio of 10 (=20 dB voltage gain) ahead of the PAT-5's phono preamp. Remember that your PAT-5 has an active line stage with 20 dB voltage gain, so the phono preamp doesn't have to provide all of the system gain ahead of the power amp.
Click to expand...

Thank you for those numbers. Essentially, if I understand this correctly, a voltage gain of a factor of 10 to 20 will bring the output of a MC cartridge to the level of a MM one, which is what the transformers are supposed to do for you. The trick here is to get it done with virtually no noise, proper impedance to keep the cartridge happy with its termination, fully wideband frequency response, vary fast transient response and virtually no distortion.

No Biggie:stupid:

Thank you sir. You have put the task into proper perspective for me.

Shelly_D
 
Shelly, MC head amplifiers are one of the easier home-brew audio projects. Many designs have been published, and some of them use cheap power transistors to provide the gain. It seems counterintuitive, but with the extremely low output impedance of an MC cartridge, small power transistors can deliver excellent noise performance.
 
BinaryMike said:
Shelly, MC head amplifiers are one of the easier home-brew audio projects. Many designs have been published, and some of them use cheap power transistors to provide the gain. It seems counterintuitive, but with the extremely low output impedance of an MC cartridge, small power transistors can deliver excellent noise performance.
Click to expand...

Thanks for that.

I'll start looking about on google. Was thinking about using an instrumentation amp chip because the design on that is dirt simple, input, output and 1 resistor for gain, but this might be a better solution as the chip I was thinking of using may not perform well at low gain.

Shelly_D
 
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