Suggested Dynaco PAS mods

the thermionic watercooler

Suggested Dyna PAS Mods

Postby Robert Cham » Fri Mar 06, 2009 2:44 pm

I've been out of the loop here, but I have lots of responses to what's been written here, so here goes. David Voorhis' "Last PAS" mods from several '70s Audio Amateurs is still my favorite. This was published in a version with tome controls as well. Norman Koren's mods are well respected, but I didn't get around to building them, at least partially because I find cathode followers to "choke" the sound in the same way that some of you feel negative feedback does. FWIW, I bought some FM3 cards from the person referred to as producing the Koren cards. Very nice product!

Several years ago, I was facing the need for low output impedance as I wanted to use Nelson Pass' passive crossover before a pair of Stereo 70s to drive my Tympani Ibs. Not only was the load 5K, but the situation was compounded by the use of a subwoofer in parallel. I bought a Phono Pre kit and a Grounded Grid kit from Bruce Rozenblitt's Transcendent Sound.

The Phono Pre was great feeding the Last PAS. The sound stage really opened up and bloomed compared to the Last PAS, which had been a vast improvement over a stock PAS. Then I finished the Grounded Grid pre. I had great hopes as grounded grid radio transmitters are superior to others, except in efficiency, especially in harmonic distortion. What a disappointment! The sound stage shrank to less then it had been with the Last PAS using it's own phono section.

Now I'm close to completing an Aikido dual mono kit with regulated power supply. Output impedance will be about 200 Ohms. We''l see how that sounds. If that doesn't hack it, next will be the four sections of 6N30 in parallel, ala Conrad Johnson.

Some comments on Jukingeo's posting about "thin" sound and other matters.I have been a full time professional recording engineer for twenty of the last forty years. Only part time for most of the rest of those years as I have earned my living as a radio engineer. There were also several years working as an engineer for API Audio Consoles, which made some of the best in the world. I say this only to assure that I do know what I'm talking about both in regard to recording equipment and practices.

Equipment and practices are generally judged by how similar the product is to live acoustic sound. Good equipment and practices can indeed produce good recordings of amplified instruments and voices. The problem is that there can not be a standard to compare them to, as the sound is pre-distorted by the amplifiers, speakers and judgement of the FOH mixing engineer.

Fewer mics by the way can often yield a superior recording because of the lack of interaction between the mics which can cause cancellation and other wonderful phase anomalies. The best stereo recording are made with two co-incident mics, IMHO.

The quality of recorded sound probably peaked in the '70s! After that home studios started to proliferate. It was very good that more people had access to recoding facilities as the home studio craze boomed. What was,and is unfortunate is that they do not have access to trained recording engineers, as well as the great microphones those big studios owned, and they are so unenlightned as to believe that they do not need them!

When it comes to the quality of non-acoustic recordings, the only real standards are one's taste. It sounds as though Geo's taste is for the heavily compressed, mixed with a "smiley" curve of '90s, club music. Perhaps what he needs is a in line compressor and a graphic equalizer set to boost both highs and lows. This is typically the taste of DJs who have abused their hearing. JBL speakers tend to be favored by this crowd as well.

I'm really not trying to put you down, but what you describe as your preferences in recordings really do sound like those of FOH sound guys , who spent too many years monitoring at too high levels.

BTW, it is my understanding that the design of the PAS filament circuit was one of saving money and extending tube life. I believe it was Ed Dell back in the seventies, who did a little project, raising filament voltage in the PAS circuit and measuring distortion. Distortion kept decreasing as the filament voltage increased. IIRC, he only went up to 15.5 V in the interests of tube life.

Bob Cham
Robert Cham
 
Posts: 29
Joined: Mon Oct 22, 2007 3:35 pm
Location: Houston, TX

Postby jukingeo » Mon Mar 09, 2009 7:04 am

Robert Cham wrote:I've been out of the loop here, but I have lots of responses to what's been written here, so here goes. David Voorhis' "Last PAS" mods from several '70s Audio Amateurs is still my favorite. This was published in a version with tome controls as well. Norman Koren's mods are well respected, but I didn't get around to building them, at least partially because


I am not familiar with the "Last PAS" mods but I will look into them. I really never had pre-amps that had cathode followers in the output, but I have had a couple of pre-amps that used a cathode follower to drive the REC out for the connection to a tape deck. Many of these pre-amp stages were either part of a preamp-amp combo or an integrated amp. Just about all of the stand alone pre-amps I messed around with (including the PAS) have had direct outputs. As I said above, this is fine if you are going to connect to a tube amp all the time. But what about the occasional SS amp hook up (for comparisons), or even a tube amp that has a lower than normal input impedance?

I find cathode followers to "choke" the sound in the same way that some of you feel negative feedback does. FWIW, I bought some FM3 cards from the person referred to as producing the Koren cards. Very nice product!


You weren't the first person to mention that. There seems to be many out there that are totally against the cathode follower and I read this in my early tube research. It seems that the only 'simple' tube preamp stage that I found 'attractive' was the SRPP design. I did quite a bit of research on it and around that time bought the Bruce Rosenblit book. Low and behold he had an SRPP variant line stage in there and I built it. I added a loudness compensation as SOME form of tone control and I liked the pre-amp a lot. But this time around I was looking to do something a bit more advanced, but yet still use simple circuits. One must was to have RIAA Phono ability as well as tone controls. Since I already have a couple PAS pre-amps, this seems like a great candidate for my project.


I bought a Phono Pre kit and a Grounded Grid kit from Bruce Rozenblitt's Transcendent Sound.

The Phono Pre was great feeding the Last PAS. The sound stage really opened up and bloomed compared to the Last PAS, which had been a vast improvement over a stock PAS. Then I finished the Grounded Grid pre. I had great hopes as grounded grid radio transmitters are superior to others, except in efficiency, especially in harmonic distortion. What a disappointment! The sound stage shrank to less then it had been with the Last PAS using it's own phono section.


Not familiar with a grounded grid design for a pre-amp, but I have seen the design used in amp splitter stages that have gain.

Now I'm close to completing an Aikido dual mono kit with regulated power supply.


I have heard of this pre-amp a couple of times and I was just starting to look into it. I think it may be a good design for a future even more 'advanced' project.

Equipment and practices are generally judged by how similar the product is to live acoustic sound. Good equipment and practices can indeed produce good recordings of amplified instruments and voices. The problem is that there can not be a standard to compare them to, as the sound is pre-distorted by the amplifiers, speakers and judgement of the FOH mixing engineer.


That is true. With a lack of a standard is the reason why recordings are all over the place. Especially with that last part. As there ARE certain equipment standards and if in good order will produce good results time after time. The FOH mixing engineer is a larger variable and is usually the first thing I point the finger at. BUT there are the instances where the said standard equipment may NOT be up to par. But if that is the case I wouldn't make a lousy recording. But I have instances where I DIDN'T have access to the right equipment and still made a good recording. I remember one time clearly when I couldn't get access to my favorite AKG C414 mic and ended up having to use a standard SM58. Surprisingly with a bit of tweaking, I was able to have a successful recording.

The quality of recorded sound probably peaked in the '70s!


Agreed, late 70's to late 80's had excellent recordings.

After that home studios started to proliferate. It was very good that more people had access to recoding facilities as the home studio craze boomed. What was,and is unfortunate is that they do not have access to trained recording engineers, as well as the great microphones those big studios owned, and they are so unenlightned as to believe that they do not need them!


And worse with today's digital electronics and the lack of know how to use the equipment properly can yield horrible recordings indeed.

It sounds as though Geo's taste is for the heavily compressed, mixed with a "smiley" curve of '90s, club music. Perhaps what he needs is a in line compressor and a graphic equalizer set to boost both highs and lows. This is typically the taste of DJs who have abused their hearing. JBL speakers tend to be favored by this crowd as well.


This is where I totally disagree with you.

Yes, I was a DJ in that time period, but I used hearing protection and my hearing today is still very good to excellent. However, I ALSO am a professionally trained audio engineer AND technician. I never played my music excessively loud either (even in a venue). No I didn't use a smiley curve on all my EQ settings I usually try to practice 'cut eq' when possible. I prefer parametric to graphic EQ (but that is obviously too complex for the simple tube pre-amp I would like to build). I DO NOT like heavily compressed sound, even though I do like the music styles from the period. I like music that has a groove that you can bob your head to and dance to. You can't really do that with Jazz and Classical. While it is true that these styles of music were/are recorded better and have better engineers, it isn't my cup of tea. I just happen to be interested in good 'ole Rock & Roll and Dance Club music.

In terms of speaker choice, this is where you are wrong about me again. I NEVER used JBL speakers in any of my pro-rigs. THERE I agree with you on. I can't stand the sound of those speakers for either home or pro use.

What I used in my pro rigs were: EAW, Community, Meyer, and EV.

For home use I have used Paradigm, Pinnacle, Klipsch, & Cambridge Soundworks. (Velodyne or M&K for subwoofers).

So as you can see, no JBL speakers.

I'm really not trying to put you down


Really? Ya' could have fooled me.

Just because I don't like Jazz or Classical shouldn't be cause to slap me with a stereotypical label that I prefer or listen to bad recordings. Not ALL recordings from the 'night club' era are bad. Yes, a good many ARE, but there are some excellent recordings from the era. But even so, there are some bad recordings of very good songs out there and some minor eq tweaking is all that is needed to make those recordings a bit easier to listen too. That is the primary reason why I want tone controls. I illustrated the Rolling Stones "Gimme Shelter" example above, so I will not repeat it again here.


but what you describe as your preferences in recordings really do sound like those of FOH sound guys, who spent too many years monitoring at too high levels.


Again, I am an exception to the rule. As I said even though I was an FOH guy myself, I always took care of myself and my hearing. Despite the content of music I preferred to work with, I never had to BLAST the sound systems I used. I ALWAYS preferred quality to quantity. Using the right system for the job is always key.

Also the fact that I would like to create a tube amp and prefer the tube sound should have been your first indication that I am an exception to that rule. After all, if I indeed did fit your JBL speaker owning/smiley EQ curve loving stereotype then would I even bother with a tube amp design? No, I don't think so. I would be out there trying to find a way to squeeze a 1000 watt per channel sound system in my living room rather than something that puts out a mere 10 watts if that were the case.

Sorry, I didn't mean to get out of hand up above there. But I just feel that I shouldn't be condemned for not being the 'norm' and wanting to listen to rock, disco and dance music through a SE tube amp. I just can't find my 'groove' with Jazz or Classical music, sorry, my musical tastes are just not built that way. I prefer to listen to the stuff I grew up (and/or worked) with. While your stereotypical remarks may be spot on for the average person that likes club music. I am far off from that mark. As I said, I wouldn't be even here in this forum if that was the case.

BTW, it is my understanding that the design of the PAS filament circuit was one of saving money and extending tube life. I believe it was Ed Dell back in the seventies, who did a little project, raising filament voltage in the PAS circuit and measuring distortion. Distortion kept decreasing as the filament voltage increased. IIRC, he only went up to 15.5 V in the interests of tube life.

Bob Cham


I probably more than likely will have to resign the power supply. I know that SRPP designs do need a good power supply and the original power supply in the PAS really isn't up to par.

Geo
Exit Light, Enter Night, Take My Hand...We're off to Never Never Land.
jukingeo
 
Posts: 198
Joined: Wed Jan 21, 2009 11:45 am

Previous

Return to diy hifi

Who is online

Users browsing this forum: Google [Bot] and 0 guests