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[Patrick Tracy]

Yawn,,,,

Should I post a funny cat picture?

[Steve M Smith]

I'll bet dollars to donuts that the rat shack pre is using crappy lousy tolerance cheap capacitors in the EQ playback network.

I would bet three shillings to a bread roll that most people couldn't tell the difference in a listening test.


Steve.

[Bob Leonard]

Yawn,,,,  I used to design high end (IMO) phono preamp (kits) back in the '70s-'80s. A good preamp would follow the RIAA spec to within a small fraction of a dB. Another subtle point if you want to properly terminate the preamp input, the source impedance of a typical MM cart was around 1-1.5k with some inductance.

After decades of designing preamps I came to the conclusion that customer behavior wrt to proper capacitance termination (for the specific cartridge), made a larger difference than my 1% resistors and 2% caps in the EQ network.

A cheap preamp OTOH can vary, and it was not that unusual to cheat a phono input in a pinch with tone controls maxed full boost on the bass end, and full cut on the treble. RIAA EQ varies about +/-20dB wrt 1khz gain. 

JR

There are only so many op amps to go around, and that's to me the issue. Regardless of the quality of the outboard components used the quality of the bug will be the deciding factor. Like when I hand pick components for the amps I rebuild. Every amp is different and it's the pre amp that always get's the most love from me.

[John Roberts {JR}]

I would bet three shillings to a bread roll that most people couldn't tell the difference in a listening test.


Steve.

One of my frustrations back when I tried to market a premium phono preamp, was precisely that critical listeners would perform listening tests, but not control for all the other variables. I could design a preamp to be arbitrarily linear and accurate to the RIAA curve, but then reviewers who should know better, connect the accurate preamp in path with far more variable transducers like the reviewer's favorite cartridge, and reviewers loudspeaker (and listening room setup).  I even experienced dramatically different reviews from the same exact review unit. This mainly informed me about which reviewer of those two had better speakers, and turntable/cart.

JR

[John Roberts {JR}]

There are only so many op amps to go around, and that's to me the issue. Regardless of the quality of the outboard components used the quality of the bug will be the deciding factor. Like when I hand pick components for the amps I rebuild. Every amp is different and it's the pre amp that always get's the most love from me.

Not to argue about the intricacies of circuit design but I mentioned the tolerance of the capacitors in the RIAA playback EQ for a reason. Without providing a long detailed proof, allow me to state that the transfer function of a negative feedback circuit, with adequate loop gain, will accurately follow the relationship defined by the feedback network. So crappy feedback component tolerance = crappy inaccurate transfer function (of course marginal open loop gain could also introduce errors while even cheap opamps have 100dB open loop gain at LF). 

The bugs inside while perhaps marginal back in the early days, the RIAA closed loop gain drops from roughly +60dB @ 50Hz down to only some +20 db at 20kHz (and keeps falling) so the falling open loop gain of a typical "bug" can easily provide adequate loop gain margin for accurate response. If anything the modern "bugs" have gotten even better. My last very good design (mid '80s)  used a discrete JFET front end for absolute best performance. Today I would be tempted to use one of the newer off the shelf bugs with, adequately low noise, high input impedance, and tons of open loop gain.

A phono preamp is/was a precision design exercise with published (by RIAA) transfer function targets. Guitar amps OTOH are less precise exercises with gain stages often operated near the limits of their open loop gain. Device variation is expected to make larger unit to unit variation, and even things like internal lead dress can alter the sound character. That is the nature of guitar amps that are intended to make original sounds, not accurately reproduce pre-recorded sounds.   

JR

[Steve M Smith]

A phono preamp is/was a precision design exercise with published (by RIAA) transfer function targets. Guitar amps OTOH are less precise exercises with gain stages often operated near the limits of their open loop gain.

That's why I build valve (tube) guitar amps (and microphone pre-amps) and not RIAA phono pre-amps!!


Steve.

[John Roberts {JR}]

That's why I build valve (tube) guitar amps (and microphone pre-amps) and not RIAA phono pre-amps!!


Steve.

Microphone preamps are also a precision design exercise with an even simpler transfer function target, "straight wire with gain". Microphones do vary all over the place, preamps don't, or should not. IMO.

If you have the luxury of selecting between different nonlinear preamps to add color to an audio path enjoy.

JR

[Steve M Smith]

I have found that a good quality microphone transformer with a very simple triode amplifier after it gives good results.

This is not a business, just something I make for friends.  One of them is a session drummer who now takes my pre-amps with him to use for recording and won't play through anything else.

I have the option of adjusting things until they like the sound and the advantage of not having to guarantee that it will meet any specifications.

I doubt that they are as linear as a high end design and I have never tested them like that.

My drummer friend also has a nine channel, all valve mixing desk which I built for him.  It is just for mixing down from tape or hard drive and has no mic pre-amps in it and no eq.  Just a triode amp with a cathode follower buffer, two inserts, four auxiliary  sends and L/R sends via a pan control per input and a stereo mix output.

This suits the way he works perfectly but would probable be inappropriate for most people.

Nineteen valves in all (ECC82 and ECC83 / 12AU7 and 12AT7).  The most I have ever built into one unit!


Steve.

[Bob Leonard]

Not to argue about the intricacies of circuit design but I mentioned the tolerance of the capacitors in the RIAA playback EQ for a reason. Without providing a long detailed proof, allow me to state that the transfer function of a negative feedback circuit, with adequate loop gain, will accurately follow the relationship defined by the feedback network. So crappy feedback component tolerance = crappy inaccurate transfer function (of course marginal open loop gain could also introduce errors while even cheap opamps have 100dB open loop gain at LF). 

The bugs inside while perhaps marginal back in the early days, the RIAA closed loop gain drops from roughly +60dB @ 50Hz down to only some +20 db at 20kHz (and keeps falling) so the falling open loop gain of a typical "bug" can easily provide adequate loop gain margin for accurate response. If anything the modern "bugs" have gotten even better. My last very good design (mid '80s)  used a discrete JFET front end for absolute best performance. Today I would be tempted to use one of the newer off the shelf bugs with, adequately low noise, high input impedance, and tons of open loop gain.

A phono preamp is/was a precision design exercise with published (by RIAA) transfer function targets. Guitar amps OTOH are less precise exercises with gain stages often operated near the limits of their open loop gain. Device variation is expected to make larger unit to unit variation, and even things like internal lead dress can alter the sound character. That is the nature of guitar amps that are intended to make original sounds, not accurately reproduce pre-recorded sounds.   

JR

JR,
Please don't confuse my statement concerning quality components with the precision of said components. High quality components are just that, regardless of degree of precision.
 
I'm fully aware of the desired goal concerning the design of a pre amp, and I can appreciate the overall increase in quality of the majority of the off the shelf pre amp packages which has been achieved over the decades. However, please be aware that the goal when building, designing, or rebuilding the first gain stages of any guitar amplifier is to also provide the following gain stages with a quality distortion free signal. Granted the majority of most guitar and high end audio components is built around the all too common 12AX7, however, there are at least a dozen or so suitable substitutes I can use in it's place.
 
My other statement concerning component choice and quality has nothing to do with component precision. It has to do with the component tonal factors, which I can guarantee exist. The ability to tailor these circuits to specific tonal needs keeps me away from the "bug" making me unqualified to design or alter those solid state circuits. However, this is not the case with tube circuits, which to me, and based on the ability to modify those circuits well beyond the original design, makes those type circuits more appealing overall.
 
I find it very strange this has become the discussion as I have a Labsters Fender "The Twin" up on the bench as we speak. The problem? Overcoming the poor sound quality and distortion introduced through the solid state portion of the pre amp circuit. Not one of Fenders best designs.

[Patrick Tracy]

Well, the fun has been beaten right out of this little diversion.

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