PM1000 Rack FINISHED +Pics & info

The Odd Coil · May 26, 2006

I had a few problems with the module going into high-freq oscillation when the treble control is turned to + boost.

You will hear it "Pop on" if so.

Otherwise it is just noise in the circuit that is being amplified by the treble boost. Shielding will not reduce/increase hiss that much. One transistor, if it is off value and the idle current is high, will make hiss. You can make a sound sniffer by a probe with a DC blocking cap into some amp and speak. then trace throung a module/circuit beg to end and you will find where the hiss starts.

Have the hiss making circuit on, but listen to it through a probe into another amp/speaker...

soundguy · May 26, 2006

[quote author="The Odd Coil"]

A. Tantalum caps are bad.

[/quote]

thats a little general, dont you think?

dave

Larrchild · May 26, 2006

These black cubes have tantalums in them.

They do ok.

The Odd Coil · May 26, 2006

I love the pic of the guy with the slide rule. I wish I had more time to chat as there are so many great brains out there. (and here)

Larrchild, are the Tants in the modules in the signal path or just used as snubbers/control in the operational part of the circuit?

I am always in tube gear and just now working on some D-Amp, (discrete amp) projects and not familar with this area. I usually am building all tube stuff so I have no time to have fun in other areas like transistor or D-Amp modules.

I am now building a tube front end, Foresell 992 diff amp and output unit for production sample and that is a balanced differential preamp. It has now transformers, but you can switch them in if desired for sound.

I just finished a production sample that has a custom D-Amp line stage and tube front end that has a lot of options. We are testing/tuning now and the combo works pretty good.

The Tants that I am familar with are from 1970's era. I don't know if they have gone through a morphing process. When op-amps were just happening, all coupling caps, mostly tube, were lost in storage and audio dudes were scrambling to find caps. Tants made now may be different.

I always use a scope and a load to be sure. When working with tubes, I build it and test it and make notes. I would always test caps to be sure.

I worked on some discrete URIE modules a few years ago and found the engineering fantastic. Really well made and designed. I have a friend that has a studio and he got some old Fairchild transistor mike preamps. Very smoooooth sound.

The Yamaha transformers are very good also. Super good freq response.

gEO...

khstudio · May 27, 2006

Thanks for sharing that info :thumb:
Your stuff looks great to BTW.

You gave me a few things to look for & try. The top unit is killer, tants & all but the bottom channel is NOT up to par for some reason... it DOES have different transistors than the top one & a few different caps... maybe the GREEN one's, I'll check. It also has different looking inductors.

I was going to try upgrading/modding the bottom channel since I don't like it or use it as is compared to the top. Also, after using it for a few months I have a very good idea how they perform & what I like & don't like:

TO TRY:
metal films in the EQ,
replace tants with ??? - (not sure yet)
& possibly swap the transistors with a third (extra) channel I have - that has the same transistors as the top channel I like.

Now, after reading your post I'm concerned about doing this:
You mentioned a trimmer to adjust the gain of the second/line stage & I wonder if I'll have to make adjustments if I swap the transistors :?:

Kevin

khstudio · May 27, 2006

OK, my top channel has the WHITE caps & the bottom has the GREEN's in the EQ, if your right about the green caps (not being as good) then that might help match them a little better... The EQ was where I noticed the BIGGEST difference in the channels so this sounds promising. The top units EQ works pretty well but the bottom doesn't seem very defined... mushy like you said.

aren't these Poly caps already :roll:
& are the WHITE ones any better :?

Did you mean Metal/Poly-film?

Kevin