PAS 3 -- sound is too bright

I have a PAS 3 (not an "x") that was built from a kit many years ago by a now-deceased family member.  He gave it to me around 1999.  I replaced the 12AX7 tubes with new tubes from Frank van Alstine in 2000.  At some point in the early 2000s I had the power supply rebuilt by a specialist in vintage audio in San Diego.  He kept the original rectifier bridge, but he replaced the big filter cap and transformer.  The transformer he used is not a Dynaco original.  After playing with the preamp a bit back then, about 20 years ago, I set it aside.  A couple of weeks ago I brought out the PAS 3.  To my delight, the power supply still functions.  There's no hum.  

I expected a "comfortable" sound -- well-balanced and a bit soft.  But I've found that the sound is bright, as though the entire frequency spectrum is tilted up.  This isn't an RIAA problem.  I get the same effect with line-level sources, like a CD player, a Parasound phono preamp, and with the original phono input.  Also, there's an imbalance between the channels, audible on all sources.  The right channel is weak but correctable with the balance control.  

My immediate solution for the bright sound has been to use the tone controls -- cut the treble, boost the bass.  But I'm wondering if there's a common problem with these preamps that causes this brightness, perhaps as the tubes age, and what I might do to address it.

Calypte wrote:I have a PAS 3 (not an "x") that was built from a kit many years ago by a now-deceased family member.  He gave it to me around 1999.  I replaced the 12AX7 tubes with new tubes from Frank van Alstine in 2000.  At some point in the early 2000s I had the power supply rebuilt by a specialist in vintage audio in San Diego.  He kept the original rectifier bridge, but he replaced the big filter cap and transformer.  The transformer he used is not a Dynaco original.  After playing with the preamp a bit back then, about 20 years ago, I set it aside.  A couple of weeks ago I brought out the PAS 3.  To my delight, the power supply still functions.  There's no hum.  

I expected a "comfortable" sound -- well-balanced and a bit soft.  But I've found that the sound is bright, as though the entire frequency spectrum is tilted up.  This isn't an RIAA problem.  I get the same effect with line-level sources, like a CD player, a Parasound phono preamp, and with the original phono input.  Also, there's an imbalance between the channels, audible on all sources.  The right channel is weak but correctable with the balance control.  

My immediate solution for the bright sound has been to use the tone controls -- cut the treble, boost the bass.  But I'm wondering if there's a common problem with these preamps that causes this brightness, perhaps as the tubes age, and what I might do to address it.

There is one thing that could cause "bright", the loading from the power amp. If the input impedance
is less then about 250k it will loose bass. What is the PAS connected to ?

As for channel imbalance, a worn volume pot is a likely candidate. There is 250k alps
pots available that will fit just fine. Don't settle for anything but 250k.

Calypte wrote:.  But I've found that the sound is bright, as though the entire frequency spectrum is tilted up.  This isn't an RIAA problem.  I get the same effect with line-level sources, like a CD player, a Parasound phono preamp, and with the original phono input.  Also, there's an imbalance between the channels, audible on all sources.  The right channel is weak but correctable with the balance control.  
.

Your description of a unit that sat for 20 years and exhibit these faults is symptomatic of age related failures.

The first thing I would do is replace all caps in the signal pathway.  Your symptoms are classic for caps that have reduced value or near death.  Caps are high pass filters.  If the coupling caps lose value through internal decay then they could easily raise the rollover frequency into the audio bandpass, thus killing the lower frequencies.  The same goes for your channel that is losing signal strength.  Eventually the cap will cease passing a signal or the rollover frequency is so high that most of the audio bandpass is suppressed.  Your preamp should get new coupling caps as a starting point. Personally, I would replace all the caps with modern equivalents. The newer caps are higher quality, anyway, and will probably improve the sound when it is finally healthy.

A second possibility is dirty switch contacts (your imbalance is a classic symptom of this possibility), especially the input selector wafer switch.  Corrosion and crud on those contacts creates resistance and capacitance, another form of high pass filter and signal attenuation.  I would acquire some non-residual contact cleaner and spray those contacts while working the switch to mechanically remove the crud.  Do this also to all of the slide switches, too, and clean all of the connectors at the back of the amp. For those connectors, you can get kits to replace the plug board with a new units that has gold plated contact surfaces.

Many units came into my audio store with the exact symptoms you describe, usually a channel failing.  Most were repaired with a simple cleaning of contacts.

Sorry. I should've mentioned the power amp. Audiosource AMP3. I otherwise use it for my back channels in a 7.2 setup. I'm just doing plain old two-channel stereo, no sub, to test the PAS 3. Speakers are Hsu Research HB-1 Mk 2. The Hsu speakers have not been particularly bright speakers when used with other equipment.

LGilbert wrote:

Calypte wrote:.  But I've found that the sound is bright, as though the entire frequency spectrum is tilted up.  This isn't an RIAA problem.  I get the same effect with line-level sources, like a CD player, a Parasound phono preamp, and with the original phono input.  Also, there's an imbalance between the channels, audible on all sources.  The right channel is weak but correctable with the balance control.  
.

Your description of a unit that sat for 20 years and exhibit these faults is symptomatic of age related failures.

The first thing I would do is replace all caps in the signal pathway.  Your symptoms are classic for caps that have reduced value or near death.  Caps are high pass filters.  If the coupling caps lose value through internal decay then they could easily raise the rollover frequency into the audio bandpass, thus killing the lower frequencies.  The same goes for your channel that is losing signal strength.  Eventually the cap will cease passing a signal or the rollover frequency is so high that most of the audio bandpass is suppressed.  Your preamp should get new coupling caps as a starting point. Personally, I would replace all the caps with modern equivalents. The newer caps are higher quality, anyway, and will probably improve the sound when it is finally healthy.

A second possibility is dirty switch contacts (your imbalance is a classic symptom of this possibility), especially the input selector wafer switch.  Corrosion and crud on those contacts creates resistance and capacitance, another form of high pass filter and signal attenuation.  I would acquire some non-residual contact cleaner and spray those contacts while working the switch to mechanically remove the crud.  Do this also to all of the slide switches, too, and clean all of the connectors at the back of the amp. For those connectors, you can get kits to replace the plug board with a new units that has gold plated contact surfaces.

Many units came into my audio store with the exact symptoms you describe, usually a channel failing.  Most were repaired with a simple cleaning of contacts.

Thank you. This is exactly the sort of thing I want to know. I'm not an electronics hobbyist. I built an FM-3 kit in 1969, but that's the sum of my experience.

Calypte wrote:Sorry.  I should've mentioned the power amp.  Audiosource AMP3.  I otherwise use it for my back channels in a 7.2 setup.  I'm just doing plain old two-channel stereo, no sub, to test the PAS 3.  Speakers are Hsu Research HB-1 Mk 2.  The Hsu speakers have not been particularly bright speakers when used with other equipment.

There is nothing wrong with your PAS !
Audiosource has an input impedance of 30 kohm according to it's manual, and that will
definitly make a PAS sound very thin.

from manualslib : manualslib

The volume pot however could be replaced. A 250k ALPS will do just fine.

peterh wrote:

Calypte wrote:Sorry.  I should've mentioned the power amp.  Audiosource AMP3.  I otherwise use it for my back channels in a 7.2 setup.  I'm just doing plain old two-channel stereo, no sub, to test the PAS 3.  Speakers are Hsu Research HB-1 Mk 2.  The Hsu speakers have not been particularly bright speakers when used with other equipment.

There is nothing wrong with your PAS !
Audiosource has an input impedance of 30 kohm according to it's manual, and that will
definitly make a PAS sound very thin.

from manualslib : manualslib

The volume pot however could be replaced. A 250k ALPS will do just fine.

Thanks. I have the original manual for the Audiosource AMP Three. I bought it about 25 yrs ago. A Best Buy was practically giving it away. I figured I could use an extra amp some day. At my old home in Temecula, CA, I had separate HT and 2-ch systems. I used it in the 2-ch with a McIntosh C29 preamp with Magneplanar 1.6 speakers. The Audiosource amp wasn't bright in that setup. I live in the mountains now, and I only have one system.

Back to Dynaco: the output spec is 1K ohms impedance. Isn't that a sufficient difference with the sufficient with the 30K ohms input on the amp? My Emotiva XPA-5, which I use for my front 5 ch, is even lower impedance: 23.5K ohms.

Calypte wrote:

peterh wrote:

Calypte wrote:Sorry.  I should've mentioned the power amp.  Audiosource AMP3.  I otherwise use it for my back channels in a 7.2 setup.  I'm just doing plain old two-channel stereo, no sub, to test the PAS 3.  Speakers are Hsu Research HB-1 Mk 2.  The Hsu speakers have not been particularly bright speakers when used with other equipment.

There is nothing wrong with your PAS !
Audiosource has an input impedance of 30 kohm according to it's manual, and that will
definitly make a PAS sound very thin.

from manualslib : manualslib

The volume pot however could be replaced. A 250k ALPS will do just fine.

Thanks.  I have the original manual for the Audiosource AMP Three.  I bought it about 25 yrs ago.  A Best Buy was practically giving it away.  I figured I could use an extra amp some day.  At my old home in Temecula, CA, I had separate HT and 2-ch systems.  I used it in the 2-ch with a McIntosh C29 preamp with Magneplanar 1.6 speakers.  The Audiosource amp wasn't bright in that setup.  I live in the mountains now, and I only have one system.

Back to Dynaco: the output spec is 1K ohms impedance.  Isn't that a sufficient difference with the sufficient with the 30K ohms input on the amp?  My Emotiva XPA-5, which I use for my front 5 ch, is even lower impedance: 23.5K ohms.

The manual also says "terminating impedance 100.000 ohm"
No it won't drive your amp. Typically PAS needs 250k or more .
See audioregenesis for a deeper discussion.

Thank you, everybody, for your input.  This PAS 3 was originally used with a Fisher tube power amp, I think an SA-1000.  I apologize, but I'm only now remembering the rest of the story.  In 2021 the deceased owner's family (he had a grown family prior to marrying my late sister-in-law) asked me to evaluate the amp, since they imagined that maybe it had great value.  I had it examined by Shelley's Stereo in Woodland Hills, CA.  The power amp had deteriorated greatly due to being stored in a series of car garages over the years.  The cost to refurbish it at Shelley's was prohibitive (IIRC about $2400), and I think Shelley's would've farmed it out to a specialist in vintage audio.  The family declined to proceed with the repair.  They weren't audiophiles, and the amp was essentially worthless junk to them.  I didn't even bother to pick it up at Shelley's after they'd evaluated it.  It would've been a 300 mile round trip to retrieve a boat anchor.  I realize that some of you in this forum would've been excited to have the opportunity to restore this Fisher power amp, and you're probably aghast that I didn't even bother to retrieve it.  If you have interest, contact Shelley's to see if they still have it.  In the meantime, WRT this PAS 3, I'm looking at two issues: I don't have a suitable power amp, and the preamp itself needs service, although it still plays.  I'll have to think about what I want to do.

I do have an additional question.  I've studied the document from audioregenisis about bypassing the tone controls.  It appears to me (right? wrong?) that this will lower the output impedance of the PAS 3 to make it more suitable for modern solid state power amps.  Most solid state amps (it appears to me) have input impedance in the neighborhood of 20K ohms or slightly greater -- i.e. nothing approaching the input impedance of the old tube amps.  Without the tone-control bypass, the only amps that are suitable partners for the PAS 3 preamp are vintage Dynaco tube amps and, perhaps, other stereo tube power amps from the 1960s (Fisher, Scott, HK, etc.).  Right? Wrong?  At least one commercial rebuilder of PAS preamps (Kenny Russell) creates a very pretty preamp, but it still needs a very high input impedance to work properly.  Right? Wrong?

Calypte wrote:I do have an additional question.  I've studied the document from audioregenisis about bypassing the tone controls.  It appears to me (right? wrong?) that this will lower the output impedance of the PAS 3 to make it more suitable for modern solid state power amps.  Most solid state amps (it appears to me) have input impedance in the neighborhood of 20K ohms or slightly greater -- i.e. nothing approaching the input impedance of the old tube amps.  Without the tone-control bypass, the only amps that are suitable partners for the PAS 3 preamp are vintage Dynaco tube amps and, perhaps, other stereo tube power amps from the 1960s (Fisher, Scott, HK, etc.).  Right? Wrong?  At least one commercial rebuilder of PAS preamps (Kenny Russell) creates a very pretty preamp, but it still needs a very high input impedance to work properly.  Right? Wrong?

You will find lots of suitable amps for the PAS, some even transistorized.

If you want to keep your poweramp and still use your PAS there is two ways :
- add a buffer between the line out and the power amp.
( build one , in complexity similar to removing the tonecontrold or obtain one in kit or ready built
even to adding one inside the poweramp where power are already availble)
- remove the tonecontrold according to audioregenesis documentation

peterh wrote:

Calypte wrote:I do have an additional question.  I've studied the document from audioregenisis about bypassing the tone controls.  It appears to me (right? wrong?) that this will lower the output impedance of the PAS 3 to make it more suitable for modern solid state power amps.  Most solid state amps (it appears to me) have input impedance in the neighborhood of 20K ohms or slightly greater -- i.e. nothing approaching the input impedance of the old tube amps.  Without the tone-control bypass, the only amps that are suitable partners for the PAS 3 preamp are vintage Dynaco tube amps and, perhaps, other stereo tube power amps from the 1960s (Fisher, Scott, HK, etc.).  Right? Wrong?  At least one commercial rebuilder of PAS preamps (Kenny Russell) creates a very pretty preamp, but it still needs a very high input impedance to work properly.  Right? Wrong?

You will find lots of suitable amps for the PAS, some even transistorized.

If you want to keep your poweramp and still use your PAS there is two ways :
- add a buffer between the line out and the power amp.
( build one , in complexity similar to removing the tonecontrold or obtain one in kit or ready built
even to adding one inside the poweramp where power are already availble)
- remove the tonecontrold according to audioregenesis documentation

Please give me a suggestion for a kit for the buffer.

A quick search on ebay gives buffer

I have no detailed knowledge of this item. It is more complicated then needed, partly since it
needs to "pump up" B+ from 12V AC , but also the circuit.
What could be all you need is a cathode follower, one 12AU7 would do. Unfortently
you cannot tap the PAS for power you need power from elsewhere.

Another FET based : FET buffer
This needs only + - 15 V which you could tap your power amp , and possibly even mount inside
the power amp.

I am absolutely unequipped to argue any of the points presented by the electronics experts in this forum.  However -- some of my reading elsewhere says that a "rule-of-thumb" for impedance matching says that the input of the amp should be at least 10x the output impedance of the preamp. The specs for the PAS 3 give its output impedance as 1,000 ohms.  The input impedance of my Audiosource AMP Three is 20,000 ohms.  By the "rule-of-thumb," this should be more than enough.  I'm inclined to think that replacing the caps and, perhaps, bypassing the tone controls, will tame the sound much more effectively than using another power amp or buffer amp.

Calypte wrote:I am absolutely unequipped to argue any of the points presented by the electronics experts in this forum.  However -- some of my reading elsewhere says that a "rule-of-thumb" for impedance matching says that the input of the amp should be at least 10x the output impedance of the preamp. The specs for the PAS 3 give its output impedance as 1,000 ohms.  The input impedance of my Audiosource AMP Three is 20,000 ohms.  By the "rule-of-thumb," this should be more than enough.  I'm inclined to think that replacing the caps and, perhaps, bypassing the tone controls, will tame the sound much more effectively than using another power amp or buffer amp.

You better read the audioregeneresis page and try to understand it.

Again -- thank you for the advice and help.  I've decided that, at least for now, I will do nothing significant with this PAS 3.  The preamp was a gift to me in 1999.  It had been unused for many years before then, and it has been unused since the mid-2000s, except for these past few days.  There's nobody alive who would be offended or care if I sold it.  Nobody outside this forum even knows that I have it.  It's not part of my main system.  I can tame the brightness and correct the channel imbalance with the controls.  I bought new tubes in 2000.  During the 2000s I had the power supply rebuilt.  I've been unable to locate documentation of who did the work or how much it cost -- which is odd, since I save everything.  The tubes and power supply are my only expenditures.  The preamp is quiet.  I've had fun playing with it.  It sounds very nice as it is.  But I've taken to heart a comment by Bob Latino from Jul 5, 2011, about the PAS 3.  Owning and using the PAS 3 is an exercise in nostalgia.  I remember when they were new.  I remember the ads in Hifi/Stereo Review and High Fidelity at a time when even a $69.95 kit was beyond my means.  But the design and most of the parts of this PAS 3 are at least 60 years old.  No amount of money will make this Dynaco PAS 3 equal or superior to a new tube preamp with modern circuitry and low output impedance, at least not if it's to continue being a real Dynaco PAS 3.  Going forward, I've been looking at the VTA preamps, Schiit Audio Freya+, and van Alstine Transcendence 10RB.