John Curl's Blowtorch preamplifier part II - Page 1307

EUVL · 27th June 2011, 04:34 AM

john curl · 27th June 2011, 06:31 PM

Well, back to 'work'.
It is still a mystery to me as to why some changes in audio design are as important as they are, at least to me (and my customer base).
There may not be any 'measurable' difference, but there is an audible difference. This has been happening for many decades to me, in fact, it often drives my design 'improvements' over the recent years. This might be better, (or different) passive parts, better grounding and transformer phasing, high speed diodes, better solder and circuit board materials, more 'ideal' passive connectors, and finally, better quality wire. Cleanliness in construction is another factor.
Now, a little bit of history:
Back when I started in electronics, over 50 years ago, I built kits with marginal solder, 'doorbell' wire that easily melted if overheated, and a soldering GUN that was huge, by today's standards. The kits worked, did what they did, OK, and worked for many years. In fact, I bet that if I got a couple of these kits out of storage, and carefully reformed the caps, they would work essentially as specified.
Yet, I would not build them like that, today, at least not audio designs. Why, is because of my accumulated experience with what works and what fails to meet expectations in audio.
Let me jump ahead 10 years, to 40 years ago, when I was assigned a project to make a solid state mixing board for the Grateful Dead. This was a combined effort with my colleague (a former boss, Ron Wickersham, still with Alembic Inc.)
I selected an 'advanced' IC that 'ran rings' around the popular 741, called an HA-911, a military part, made with dielectric isolation, an expensive process at the time.
The price was daunting, perhaps $5 each, or about an hour's pay for an engineer at the time. We got lucky and got 1000 rejects (tempco) in flat pack that cost us only 0.25 each. A challenge to test, and they had to be hard wired into the circuit, but still a good deal. In those days, we used coupling caps just about everywhere. We were used to it, and any DC offsets could be taken out. The problem was that we selected a 2.2uF CERAMIC CAP that Ron found surplus. These parts, at the time, were relatively expensive, but small and non-polar. We also used Tantalum caps for higher values, and Allen Bradley mil grade pots and baked carbon resistors throughout the entire unit. We built it, tested it for distortion, using my trusty Heathkit IM analyzer, able to resolve 0.005% distortion, looked for oscillations, etc, with my trusty 80 MHz Tek Scope, and released it for audition with the band.
They started to use it, and put away their vacuum tube mixers, except as back-up. Well, after awhile on the road, they damaged the solid state mixer, and had to revert to the tube mixers in reserve. They NEVER went back to solid state mixer, they could EASILY hear the difference between an OPEN LOOP TUBE MIXER and my solid state IC based mixer, and soon I was out of a job.
Now, what did I do wrong? For the time, NOTHING! Yet, the solid state mixer, 'sucked'. During my latent period of being on unemployment, I attempted to 're-think' what it took to make really good solid state electronics.
About one and 1/2 years later, in late 1972, I was re-hired by the GD to build 'The Wall of Sound' system. Now what? What could I do that was different and perhaps be successful? This is where the topologies that you see in the Levinson JC-2 were born, coupled with some a Matti Otala's ideas, of high open loop bandwidth and high slew rate were added, and guess what? SUCCESS! Yes, and not only with the GD, but with Mark Levinson, as well, then an up-in-coming maker of high end audio electronics.
And people wonder why I am so skeptical of traditional op amps. But it was not just the op amp that was the problem, it was ALSO the coupling caps, but I had not yet measured this problem with my test equipment. About 1 year later, in 1974, Tek showed me the way to measure caps, and ceramics, (except for a few small value and expensive ones) are really SCARY, when you actually see their non-linearity. more later

scott wurcer · 27th June 2011, 11:26 PM

Quote:

Originally Posted by john curl

They started to use it, and put away their vacuum tube mixers, except as back-up. Well, after awhile on the road, they damaged the solid state mixer, and had to revert to the tube mixers in reserve. They NEVER went back to solid state mixer, they could EASILY hear the difference between an OPEN LOOP TUBE MIXER and my solid state IC based mixer, and soon I was out of a job.

Was that with 50% or 100% overdrive?

john curl · 27th June 2011, 11:37 PM

What are you talking about? You have never heard of an Ampex MX-10 mixer? Before your time, and mine, even, but they sound pretty good.

Joachim Gerhard · 28th June 2011, 12:24 AM

Here it is :
http://recordist.com/ampex/pdf/mx10.pdf

quite cute.

john curl · 28th June 2011, 12:31 AM

Thanks Joachim.

scott wurcer · 28th June 2011, 12:31 AM

Quote:

Originally Posted by john curl

What are you talking about? You have never heard of an Ampex MX-10 mixer? Before your time, and mine, even, but they sound pretty good.

My experience with live rock music is not hi-fidelity formost. We hosted an early Aerosmith concert, 11+.

john curl · 28th June 2011, 12:47 AM

The GD, with Ron Wickersham and Bear (Owsley) used the MX-10's, bypassing the second stage in some way. This is how they mixed the vocals for a few years. My job was to replace the MX-10's with something, solid state, that sounded just as good, yet was more reliable on the road, the tubes being sensitive to vibration damage. We did our master recording direct, from a splitter in the mike lines, and direct boxes.
Not all rock and roll is just overload and distortion. One should come to know this, if objectivity is to be maintained.

scott wurcer · 28th June 2011, 01:01 AM

Quote:

Originally Posted by john curl

The GD, with Ron Wickersham and Bear (Owsley) used the MX-10's, bypassing the second stage in some way. This is how they mixed the vocals for a few years. My job was to replace the MX-10's with something, solid state, that sounded just as good, yet was more reliable on the road, the tubes being sensitive to vibration damage. We did our master recording direct, from a splitter in the mike lines, and direct boxes.
Not all rock and roll is just overload and distortion. One should come to know this, if objectivity is to be maintained.

Show me a serious PA speaker with less than several percent distortion at contractualy specified SPL's. That is, the PA shall provide 120dB at X feet the fidelity is an N/A.

SY · 28th June 2011, 01:22 AM

I've been to perhaps a dozen GD shows. The sound was good by the standard of lousy arena sound.

28th June 2011, 01:22 AM	#13070
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	I've been to perhaps a dozen GD shows. The sound was good by the standard of lousy arena sound. __________________ If there's a sucker born every minute, where do the rest of them come from?

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27th June 2011, 04:34 AM	#13061
EUVL diyAudio Member Join Date: Oct 2003

27th June 2011, 06:31 PM	#13062
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Well, back to 'work'. It is still a mystery to me as to why some changes in audio design are as important as they are, at least to me (and my customer base). There may not be any 'measurable' difference, but there is an audible difference. This has been happening for many decades to me, in fact, it often drives my design 'improvements' over the recent years. This might be better, (or different) passive parts, better grounding and transformer phasing, high speed diodes, better solder and circuit board materials, more 'ideal' passive connectors, and finally, better quality wire. Cleanliness in construction is another factor. Now, a little bit of history: Back when I started in electronics, over 50 years ago, I built kits with marginal solder, 'doorbell' wire that easily melted if overheated, and a soldering GUN that was huge, by today's standards. The kits worked, did what they did, OK, and worked for many years. In fact, I bet that if I got a couple of these kits out of storage, and carefully reformed the caps, they would work essentially as specified. Yet, I would not build them like that, today, at least not audio designs. Why, is because of my accumulated experience with what works and what fails to meet expectations in audio. Let me jump ahead 10 years, to 40 years ago, when I was assigned a project to make a solid state mixing board for the Grateful Dead. This was a combined effort with my colleague (a former boss, Ron Wickersham, still with Alembic Inc.) I selected an 'advanced' IC that 'ran rings' around the popular 741, called an HA-911, a military part, made with dielectric isolation, an expensive process at the time. The price was daunting, perhaps $5 each, or about an hour's pay for an engineer at the time. We got lucky and got 1000 rejects (tempco) in flat pack that cost us only 0.25 each. A challenge to test, and they had to be hard wired into the circuit, but still a good deal. In those days, we used coupling caps just about everywhere. We were used to it, and any DC offsets could be taken out. The problem was that we selected a 2.2uF CERAMIC CAP that Ron found surplus. These parts, at the time, were relatively expensive, but small and non-polar. We also used Tantalum caps for higher values, and Allen Bradley mil grade pots and baked carbon resistors throughout the entire unit. We built it, tested it for distortion, using my trusty Heathkit IM analyzer, able to resolve 0.005% distortion, looked for oscillations, etc, with my trusty 80 MHz Tek Scope, and released it for audition with the band. They started to use it, and put away their vacuum tube mixers, except as back-up. Well, after awhile on the road, they damaged the solid state mixer, and had to revert to the tube mixers in reserve. They NEVER went back to solid state mixer, they could EASILY hear the difference between an OPEN LOOP TUBE MIXER and my solid state IC based mixer, and soon I was out of a job. Now, what did I do wrong? For the time, NOTHING! Yet, the solid state mixer, 'sucked'. During my latent period of being on unemployment, I attempted to 're-think' what it took to make really good solid state electronics. About one and 1/2 years later, in late 1972, I was re-hired by the GD to build 'The Wall of Sound' system. Now what? What could I do that was different and perhaps be successful? This is where the topologies that you see in the Levinson JC-2 were born, coupled with some a Matti Otala's ideas, of high open loop bandwidth and high slew rate were added, and guess what? SUCCESS! Yes, and not only with the GD, but with Mark Levinson, as well, then an up-in-coming maker of high end audio electronics. And people wonder why I am so skeptical of traditional op amps. But it was not just the op amp that was the problem, it was ALSO the coupling caps, but I had not yet measured this problem with my test equipment. About 1 year later, in 1974, Tek showed me the way to measure caps, and ceramics, (except for a few small value and expensive ones) are really SCARY, when you actually see their non-linearity. more later

27th June 2011, 11:37 PM	#13064
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	What are you talking about? You have never heard of an Ampex MX-10 mixer? Before your time, and mine, even, but they sound pretty good.

28th June 2011, 12:31 AM	#13066
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Thanks Joachim.

28th June 2011, 12:47 AM	#13068
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	The GD, with Ron Wickersham and Bear (Owsley) used the MX-10's, bypassing the second stage in some way. This is how they mixed the vocals for a few years. My job was to replace the MX-10's with something, solid state, that sounded just as good, yet was more reliable on the road, the tubes being sensitive to vibration damage. We did our master recording direct, from a splitter in the mike lines, and direct boxes. Not all rock and roll is just overload and distortion. One should come to know this, if objectivity is to be maintained.