dogears said:
Hey John, something you've mentioned several times was that Peavy did everything possible to eliminate trimmers.
Can you expand on how you'd do that from a design perspective? Was it primarily by component tolerance sorting / binning, or custom component ordering, or...?
Design... and tight component specifications.
For the example I mentioned of biasing up class A/B amplifier output stages, at the time Peavey was probably Motorola's largest customer for TO-3 power transistors and we had several complementary pairs in our system tightly specified to behave well at popular power points (Peavey sold lots of amplifiers). The double diode, also purchased from Motorola was tightly specified to perform well with the preferred Peavey output stage pairs. I don't know all the details of that design process because it was done before my time there (mid-80s). I felt little reason to re-invent the wheel for amplifier stages inside my powered mixers.
The trim-less class A bias was a bit of a compromise, amplifiers when cold could be a little bias starved (more on that later) but across millions of amplifier channels it was proved robust and reliable. At one point when I was working as product manager over all Peavey power amps, I inherited a heavy iron class A/B amp (PV1.3k). These amps had a less than stellar reputation for thermal stability. This is a little inside baseball about amplifier design but the susceptibility for thermal runaway is a function of thermal resistance junction to ambient.... In other words, amplifiers with marginal power dissipation capability are more likely to run-away thermally and melt down. Peavey was a sharp pencil company when it came to approving cost increases to existing products, but as product manager with my awareness of how amplifier worked, I was able to negotiate the cost increase in a new product authorization. I rebranded the PV1.3k as the PV2000, pretty much the same amp but with 4 more TO-3 power transistors. I couldn't add heatsink, but adding more power devices reduced the effective thermal resistance junction to ambient making the amp rock solid thermally. The extra devices also lowered the distortion because the individual power devices weren't working as hard but that was just gravy.
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For the "more" about starved class A bias when cold, Peavey had a modest power guitar amp that used plastic darlington output devices. That one amp model was notorious for sounding ratty on cold winter mornings if the dealer didn't warm it up before demoing it to a potential customer.
Wearing my design engineer hat, I actually came up with a slick (IMO) self biasing scheme using a LTP detecting the voltage drop across the emitter degeneration resistors to crudely regulate class A bias current irrespective of the power devices. When I told Jack Sondermeyer I could fix his amp, he said "show me"... so I did. My modified guitar amp worked but that was not enough to get Jack to redesign one of his amps that was already in production.
For me to prove my design worked I would have to design it into one of my new products for full production. The opportunity for that was a high tech, dsp based, Karaoke top box I was tasked with overseeing. This SKU had a modest 35W power amp, perfect for proving my design, BUT..... at the time my political stature was compromised so I was forced to report to the digital engineering director (he was a much better politician than me :
). He refused to gamble on using my unproved amp design that could delay the product introduction or cause service issues if it didn't work perfectly (another reason to never work for a boss). Ironically that Karaoke SKU suffered horrendous field service problems due to a Japanese cassette transport that would jam and lock up... It took me the better part of a year trying to figure out what I did wrong that was causing the lock ups. It was the same exact transport I had used in the Peavey 4T cassette but using a cheaper 2T stereo head. The Japanese vendor finally admitted that they had changed the durometer of one of the plastic gears inside, causing the lock-ups. By that time the product had earned a terrible market reputation and was irredeemable.
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Also on the topic of designing out trims, one of my several patents while working at Peavey was for a trimmer-less VCA.
US04818951 Roberts
04/04/1989 Gain control or multiplier circuits. This was a circuit design that utilized separate DC and audio frequency feedback paths to automatically bias transistors in a VCA design so unmatched transistors could be used in production. This VCA was used in the old AMR CDS-2 compressor/limiter/de-esser. The Patent was assigned to Peavey Electronics.
This was a useful to avoid factory trims and selected parts but class A VCA was not very high performance and eventually abandoned.
This was back before dbx/THAT corp freely shared their VCAs with potential competitors. This class A VCA was good enough for a cheap -10dBV comp/limiter/de-esser but just barely. Later when the good dbx VCAs were made available Peavey used THAT corp VCAs in dynamics processors (DOH).
JR
