Suggested Dyna PAS Mods
Posted: 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
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