Dynaco PAS 3 Sound Quality

[malden]

In rotation is my completely stock PAS 3 and re-capped Stereo 70. The PAS 3 has all of the original tubes which all tested good. It works flawlessly with no noise or hum. It has had no mods whatsoever.

I was listening to Beethoven's 7th on my turntable and noticed that the mid range (strings mostly) were weak and muddy. I swapped the PAS for a Kenwood solid state which has pre-out jacks and the sound was much better in the mid section, stronger with more detail.

Is this typical for a stock PAS 3? Will a re-cap help improve the sound?

 

[peterh]

Define "improve" Do you mean "true to the recording" or do you mean "sugar for the ears" ?

Changing caps will in best cast not change anything.
The most difference from the recorded real music is in the speakers. The main problem is
our hearing memory, when is the sound natural ? Or do we like the sweetened sound ?

Try some recording where you have own experience of live music.

If you decide to replace the PAS, selling it unmunged will give the best price.

 

[primosounds]

In rotation is my completely stock PAS 3 and re-capped Stereo 70. The PAS 3 has all of the original tubes which all tested good. It works flawlessly with no noise or hum. It has had no mods whatsoever.

I was listening to Beethoven's 7th on my turntable and noticed that the mid range (strings mostly) were weak and muddy. I swapped the PAS for a Kenwood solid state which has pre-out jacks and the sound was much better in the mid section, stronger with more detail.

Is this typical for a stock PAS 3? Will a re-cap help improve the sound?

If your pas3 is from the 60s or 70s even 80s, it is a minimum of 30+ years old and could be nearly 60! It would be effing amazing if a stock unit gave high fidelity performance after that many years. The phono section of a preamp is dealing with milllivolt signals and this signal can be compromised with a little bit of oxidation on the rca jacks, in the tube sockets, any place that there is a contact interface. Try cleaning those areas and see if it makes a difference. Tube strength in the phono section may also be a factor so if could try some known strong tubes there it would be an easy check.

 

[tcdriver]

Do you have a PAS 3 or PAS 3X? There is an important difference. The PAS 3 will put a DC voltage on the output of the preamp which will feed the input of your amplifier. This has the nasty effect or upsetting the bias on the 7199, which is not good and can cause significant distortion in the Stereo 70.

If you do have a PAS 3, you can simply add a couple of capacitors to eliminate the DC output.

 

[malden]

Do you have a PAS 3 or PAS 3X? There is an important difference. The PAS 3 will put a DC voltage on the output of the preamp which will feed the input of your amplifier. This has the nasty effect or upsetting the bias on the 7199, which is not good and can cause significant distortion in the Stereo 70.

If you do have a PAS 3, you can simply add a couple of capacitors to eliminate the DC output.

Yes, it's the PAS 3, not the X version. I'd say overall the sound quality is good without distortion. Listening to CD's. the mids sound ok but, without the exact same recording as the LP version, I hesitate to say that only the phono section is lacking in mid-range clarity.

 

[malden]

Define "improve" Do you mean "true to the recording" or do you mean "sugar for the ears" ?

In this particular case, I'll define "improve" for you....as I stated in my first post, PAS 3 sounds weak and muddy in the mid-range whereas the Kenwood sounds strong, clear and articulate.

Changing caps will in best cast not change anything.
The most difference from the recorded real music is in the speakers. The main problem is
our hearing memory, when is the sound natural ? Or do we like the sweetened sound ?

I've had several sets of speakers in this tiny bedroom and have settled on a pair that sound the best there. They provide a benchmark for testing other components. In this case, it's the amplification that has the biggest impact on the quality of the sound.

Try some recording where you have own experience of live music.

That's precisely what I have done

If you decide to replace the PAS, selling it unmunged will give the best price.

I agree

 

[malden]

If your pas3 is from the 60s or 70s even 80s, it is a minimum of 30+ years old and could be nearly 60! It would be effing amazing if a stock unit gave high fidelity performance after that many years. The phono section of a preamp is dealing with milllivolt signals and this signal can be compromised with a little bit of oxidation on the rca jacks, in the tube sockets, any place that there is a contact interface. Try cleaning those areas and see if it makes a difference. Tube strength in the phono section may also be a factor so if could try some known strong tubes there it would be an easy check.

Some good points. I did clean the RCA jacks but not the tube sockets, will give that a try.

 

[gadget73]

is it restored? Low heater voltage is pretty common with an unrestored version, and that can definitely affect performance. The tubes ran a little on the cool side to begin with, add some degradation of the heater supply and it gets worse.

 

[malden]

is it restored? Low heater voltage is pretty common with an unrestored version, and that can definitely affect performance. The tubes ran a little on the cool side to begin with, add some degradation of the heater supply and it gets worse.

completely stock, all original components, no mods, original tubes (tested good)

 

[airtime]

I have a PAS that had already been somewhat "munged" by the time I got it. I rebuilt it to the original circuit with fresh, upgraded parts. The sound is anything but weak and muddy. The decision any PAS owner faces is whether to keep it as original as possible or freshen the parts for sound quality.

That's a slippery slop modding up those PAS preamps. I've had a few and in the end I wondered why didn't I just start out with a new preamp and side stop all these mods.

I lean towards just getting the old PAS's cleaned up and working back up to spec. Maybe a better volume pot, tweak up the PS and some new tubes, but that's about it. After that you may as well just get a new kit and tweak it up from there. I've been down that road and it didn't end well for me several times. If I had to do it all over again that's the way I would go.

 

[shelly_d]

It sounds like you have run into the classic issue that the Stereo 70 has. Too much low bass or some DC bias on the input will cause the first stage of the amp (pentode of the 7199) to go into saturation. Eliminating that low bass (sub audible) or the DC on the input of the amp will get the amp sounding a lot better. TCDriver is on the right track. A very fine analysis of this issue can be found here:

http://www.enjoythemusic.com/magazine/manufacture/0902/index.html

I read it and it made sense to me. (I am an engineer by training and trade and read it in that light.) I found that on my installation, a Dynaco PAT 5 and a Stereo 70, all I needed was the bandpass filter he talks of. (I set mine to about 17 Hz to about 21 kHz). The muddyness you noticed went away completely. The amp sharp, clear and responsive to all my music.

 

[goldear]

The PAS three is one of the most overrated preamps of all time. This probably comes from the fact that you can modify these to sound pretty darn good. But a simple recapping is not enough to get this beast into the magical territory. Some significant circuit changes are mandatory, as well as a well regulated power supply.

I say this as somebody who ran a PAS for many years (modifying it within an inch of its life) in my system before finally upgrading to a much better sounding tube preamp. Frankly you are better off just selling it and buying a really good sounding tube preamp instead of modding it. Modding these made sense whey they could be had for nearly free back in the late seventies and early 80s, and good tube preamps sold for over a thousand bucks. But now the world is awash in 80s and 90s vintage tube preamps which will walk all over the PAS, and yet sell for pennies on the dollar. Often these can be bad for less than a stock PAS sells for today.

 

[airtime]

The PAS three is one of the most overrated preamps of all time. This probably comes from the fact that you can modify these to sound pretty darn good. But a simple recapping is not enough to get this beast into the magical territory. Some significant circuit changes are mandatory, as well as a well regulated power supply.

I say this as somebody who ran a PAS for many years (modifying it within an inch of its life) in my system before finally upgrading to a much better sounding tube preamp. Frankly you are better off just selling it and buying a really good sounding tube preamp instead of modding it. Modding these made sense whey they could be had for nearly free back in the late seventies and early 80s, and good tube preamps sold for over a thousand bucks. But now the world is awash in 80s and 90s vintage tube preamps which will walk all over the PAS, and yet sell for pennies on the dollar. Often these can be bad for less than a stock PAS sells for today.

You got it exactly right. I think that was the idea I was trying to express but failed. The PAS is a NICE little preamp but it is what it is.

 

[primosounds]

You got it exactly right. I think that was the idea I was trying to express but failed. The PAS is a NICE little preamp but it is what it is.

I mostly agree with you in that compared to modern preamps the PAS most likely will come in second comparing them head to head. But, it is not like a Yugo vs Lamborghini more like a Golf vs Beemer. The bmw will spank the vw all day but when they go into the shop whose owner is crying over the bill?

 

[gadget73]

I think I'd probably start by comparing all voltages against the PAS assembly manual just to see if you're somewhat close to operating correctly. I'd be especially interested in the heater supply voltage. The tubes might be fine, but if they don't get enough heater voltage the emission will be low and performance will suffer.

I know opinions range all over the place about how these perform, but you're never going to be able to give it an honest evaluation if it doesn't at least work correctly to start with. If its 100% and you still don't like it it, then its time for something else.

 

[shelly_d]

With all this discussion of the merits of the PAS3, I would point out that the OP's description of the issue points directly to the one real, universally acknowledged deficiency of the power amp, muddy mid-range and treble. Lets see if we can focus on helping with the problem he posted about, rather then the ones we see as issues.

Shelly_D

 

[malden]

Thank you for all of the suggestions. I will do the following:

1) not re-cap it
2) clean tube sockets and pins
3) test for DC output
4) make sure it is running within original Dynaco specs

Number 1 is easy enough, so is number 2. I have the original assembly manual but will need some help with 3 & 4

 

[gadget73]

3 is real easy. Multimeter set to DC, ground probe to chassis, the other in the center of the output RCA jack. It wants to be zero if things are right. If you get a half volt or so, the blocking capacitors will take care of it.

4 isn't too hard either. I'd have to look at the manual to confirm, but I'm certain it has a page in there with voltage readings from ground to each tube pin. Maybe on the schematic page? Its there someplace.

 

[malden]

3 is real easy. Multimeter set to DC, ground probe to chassis, the other in the center of the output RCA jack. It wants to be zero if things are right. If you get a half volt or so, the blocking capacitors will take care of it.

4 isn't too hard either. I'd have to look at the manual to confirm, but I'm certain it has a page in there with voltage readings from ground to each tube pin. Maybe on the schematic page? Its there someplace.

Awesome! Thanks

 

[malden]

I forgot...

5) add capacitors per recommendation of @tcdriver and @shelly_d

will need some guidance with that one as well