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low distortion 6 transistor preamp
low distortion 6 transistor preamp
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Old 15th July 2017, 10:19 PM   #1
jpc2001 is offline jpc2001  United States
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Default low distortion 6 transistor preamp

Here's a discrete preamp that can produce vanishingly low distortion (-120db) into a 2.2kOhm or larger load, with a 3V p-p output signal.

Why not opamps? Because this is transistor golf. Each gain stage in the preamp uses only 6 bipolars.

The first attached image is the core gain stage. The second image builds upon it into a full preamp circuit with two sets of baxandall tone controls at different cutoffs.

Why dual tone controls? If I build this, it'll be a drop-in replacement for the preamp board in a Pioneer SA-9500 mk2 which uses this dual tone setup. Values for the tone circuit are unchanged from the original Pioneer design.

Constructive criticism welcome. I'm still learning.

One nice thing about this circuit: tolerances don't matter. There are no betas or Vbe drops or resistors to match. Nothing needs to be thermally coupled.

Edit: Post 32 has final, as-built schematics. The images here were the original concept. (It turns out we can't edit attachments to the #1 post...)
Attached Images
File Type: png pre_gain_stage.png (64.7 KB, 828 views)
File Type: png preamp.png (70.8 KB, 816 views)
File Type: png preamp_tone.png (41.2 KB, 774 views)
File Type: png pre_fft.png (12.1 KB, 763 views)

Last edited by jpc2001; 15th August 2017 at 08:07 PM.
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Old 16th July 2017, 03:57 PM   #2
jpc2001 is offline jpc2001  United States
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Here's an updated gain stage and preamp, with somewhat better low-impedance performance and improved biasing of IN- in the tone control stages such that it won't affect in-band frequency response.
Attached Images
File Type: png pre_gain_stage.png (42.3 KB, 727 views)
File Type: png preamp.png (73.1 KB, 295 views)
File Type: png pre_fft.png (25.9 KB, 148 views)
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Old 16th July 2017, 07:36 PM   #3
DUG is offline DUG  Canada
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Re: post 2

Q4 and Q5 appear to be fighting each other signal polarity wise.

There is no control (even by design) of the DC output level.
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Old 16th July 2017, 10:19 PM   #4
jpc2001 is offline jpc2001  United States
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Hi Dug!

Q5 is a nearly constant current source. Q4 swings the output voltage up and down; Q3 is a buffer so that the input stage doesn't have to drive the junction capacitance of Q4. Together Q3 and Q4 work just like the VAS in a power amplifier, and Q5 works like an active VAS load.

You're right about the DC output level. Like an opamp, the core gain block doesn't control it at all, it relies on an external feedback network to close the loop and set output level at DC. It also relies on the feedback network to supply a bit of bias current into the negative input. That's what R111 through R114 are doing in the tone control stages, they dribble just enough current from the output back to the negative input to bias the negative input correctly. The value of these resistors sets the output DC voltage also. I don't love that the output voltage ends up being a function of transistor beta so I may revise that biasing before this circuit is final.

ps. It's not final yet
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Old 16th July 2017, 10:34 PM   #5
jpc2001 is offline jpc2001  United States
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This revised gain stage works better at the input of the preamp, it copes much better with a source impedance. The volume pot preceding it can present up to 25Kohm source impedance so that's a requirement.

Previous FFTs were taken with only 220 ohms of input impedance. When I raised that to 22Kohm, distortion performance fell apart, around -90db for the input stage. Yechh.

The new gain stage adds a cascode so that the input transistor never sees a voltage swing at its collector. That gets us back to -118db distortion performance when driving a 1kOhm load. Still only 6 transistors!

I'm keeping the original gain stage for the tone control sections. It requires only microamps of bias on the negative pin, which was needed to get the biasing right with the tone stages. The input stage doesn't need that, it can happily drive the 800uA of bias that the new gain stage needs on its negative input.
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File Type: png new_pre_gain_stage.png (41.3 KB, 275 views)
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Old 17th July 2017, 11:03 AM   #6
DUG is offline DUG  Canada
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So, you are designing an discrete op-amp not a discrete preamp.

OK

Q4 and Q5 still fight each other.
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Old 17th July 2017, 03:24 PM   #7
DF96 is offline DF96  England
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Quote:
Originally Posted by jpc2001
A caveat: I've only simulated this circuit.
I suspected that when I read your first sentence claiming -120dB distortion. In Spiceworld magic is possible.
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Old 17th July 2017, 05:13 PM   #8
jpc2001 is offline jpc2001  United States
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If you evaluate these gain stages as op-amps, they're not very good: PSRR is low, input offset is nowhere near 0V, input bias current is worse than most monolithic op-amps, and the output current is limited. I haven't qualified the CMRR or OLG or other parameters that you'd normally specify in an opamp. These gain stages rely on split supplies and also a ground, so they couldn't fit in a standard op-amp pinout.

Those issues could be fixed with more elements. We can have excellent distortion performance, a short bill-of-materials, or op-amp levels of generality -- choose any two.

That's why I call it a preamp, I'm just not going for op-amp generality, not trying to optimize every parameter people evaluate opamps with. These gain stages and their host circuit are designed together. Like any good relationship, they overlook each others' weak points. Or maybe it's a co-dependent relationship?
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Old 17th July 2017, 10:04 PM   #9
jpc2001 is offline jpc2001  United States
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Quote:
Originally Posted by DF96 View Post
In Spiceworld magic is possible.
What mechanism would cause these circuits to behave far worse than the sim?

My understanding is that Spice may model Early effect and junction capacitance in BJTs as more linear than reality.

That said, with the cascode added in post #5, neither Early effect nor CJC should come into play for the Q2 input transistor. Q3 sees only small AC voltages and currents and shouldn't see either effect. Q4 will see a lot of Early effect and CJC current, which is mitigated by Q3 whose only purpose is to drive Q4's CJC so the input stage doesn't have to. Spice shows this very well if you remove Q3: distortion gets much worse.

Similarly, the tone control gain stage from post #2 has only tiny AC voltages at its inputs, so Early effect and CJC effects in the IPS should be negligible. The rest of the analysis is as before.

What am I missing?

I could easily miss something if Spice doesn't catch it. I lean on the simulator for verification. And Spice is optimistic. It's the common lot of the engineer to obtain real-world measurements are a little worse than theory predicts.

But there's still nothing so practical as a good theory! My experience has been that when Spice predicts a reduction in distortion from a given circuit change, that change is usually an improvement in subjective listening, or at worst it sounds the same. That could be expectancy bias. When possible I enlist a friend to listen blind to "A" and "B" to reduce this bias.
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Old 18th July 2017, 12:02 AM   #10
Nelson Pass is offline Nelson Pass  United States
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low distortion 6 transistor preamp
This would be the part where you build one and find out.

As you point out, the models have some simplifying assumptions, and
of course the performance of parts vary between part samples.

That is not to say the result might be as good as the sim.....
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