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Old 8th September 2016, 04:27 AM   #121
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by xrk971 View Post
These transistors have Hfe in the 470 range. Isn't open loop gain on two stages 220k?
hfe is current gain. At some point in the circuit that has to be translated into voltage gain, usually by dumping all that current into a large impedance.

I haven't simulated the circuit in LTSpice, or done formal calculations, so perhaps I'm wrong. But it looks as though the relatively low collector loads (3.3k) will shunt away (waste) a fair bit of the current gain.

The complimentary output devices effectively have current-source loads, which is good for high voltage gain; but now you have a very high output impedance there, and there's nothing to buffer the output! So if you connect any reasonable load to the output, the open-loop gain of the circuit drops like a rock.

There were some truly brilliant analogue designers behind many of the wonderful "classic" op-amps we are so casual about today. IMO, the performance of even an old-school NE5532 or TLO72 is pretty hard to match with a discrete circuit.

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Old 8th September 2016, 07:06 AM   #122
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Gnobuddy View Post
There were some truly brilliant analogue designers behind many of the wonderful "classic" op-amps we are so casual about today. IMO, the performance of even an old-school NE5532 or TLO72 is pretty hard to match with a discrete circuit.

-Gnobuddy
Of course, and thatīs the point.

But armchair designers try to beat them throwing a handful of parts over the table and claiming superiority
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Old 8th September 2016, 07:19 AM   #123
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by xrk971 View Post
These transistors have Hfe in the 470 range. Isn't open loop gain on two stages 220k?
Thatīs *current* gain, not voltage gain which is implied in the "open loop" expression.
I calculate (in my mind, I do not simulate) available gain about 30/40X on the differential stage and 40X in the scond one, so we are in the order of some 1200/1400X open loop gain, 30 something dB, a FAR cry from 220k , which would have been reasonable in an old Op Amp, think 741 .
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Old 8th September 2016, 08:24 AM   #124
vzaichenko is offline vzaichenko  Russian Federation
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Transistor Preamp
Guys, in many cases, knowledgeable experienced people are posting their comments, based on either some "well known" general information, or some very rough assumptions.
You are just confusing the less experienced members, reading your comments and making decisions, based on the very rough, inaccurate information you provide.

Now, let's see what the truth is.
I'm talking about schematic from the post #7 (top picture - bjt input version, attached here as well for your reference).

OLG @ 1KHz = 80db (far from 30db, mentioned above)
THD @ 1KHz open loop = 0.03% (not bad at all for open loop with such gain)
ULGF = 1.8MHz

CLG @ 1KHz = 12db (as set by the FB network)
THD @ 1KHz = 0.00011% (not enough for somebody?)
Output impedance = 9.7 ohm (more than "low enough" for driving some 10...100 Kohm input impedance of the power amp). Don't forget - NFB reduces the output impedance.

Conclusion - perfect for the purpose.

Please ... let's try to keep more attention to detail.

Cheers,
Valery
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File Type: jpg APEX BJT PREAMP.jpg (382.6 KB, 543 views)
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Old 8th September 2016, 01:07 PM   #125
xrk971 is offline xrk971  United States
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Thanks for doing that Valery. Explains why it sounds pretty darn good. I even connected my headphones to it and seems to drive them just fine so confirms the output impedance is low.
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Old 8th September 2016, 01:08 PM   #126
vzaichenko is offline vzaichenko  Russian Federation
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Default AWire - VHex-based line stage

Just for fun - my version. The front-end from VHex amplifier, slightly adapted for line stage requirements.
Basic parameters:

OLG @ 1KHz = 95db
THD @ 1KHz open loop = 0.04%

CLG @ 1KHz = 12db
THD @ 1KHz = 0.00006%
Output impedance = 1 ohm
ULGF = 7.5MHz

No Miller compensation (allowed by current-driven VAS).
Fast and clean step response.
In fact - same grade as the one from Apex above. Key difference - I like this compensation scheme sound-wise. Ultimate clarity.

Cheers,
Valery
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File Type: jpg VHex-based-linestage-01.JPG (95.8 KB, 523 views)
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Last edited by vzaichenko; 8th September 2016 at 01:11 PM.
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Old 8th September 2016, 01:19 PM   #127
vzaichenko is offline vzaichenko  Russian Federation
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Huh ... just thought - this one can be built right on VHex+ boards - just don't install the output devices, remove the spreader and put some jumper wires
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Old 8th September 2016, 01:55 PM   #128
voltwide is offline voltwide  Ireland
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Quote:
Originally Posted by xrk971 View Post
These transistors have Hfe in the 470 range. Isn't open loop gain on two stages 220k?
No.
Open loop gain is a matter of voltage gain, not current gain.
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Old 8th September 2016, 02:35 PM   #129
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by vzaichenko View Post
Huh ... just thought - this one can be built right on VHex+ boards - just don't install the output devices, remove the spreader and put some jumper wires
I was thinking the same thing. It would be a pretty decent headphone amp too. What's max power output from KSA/KSC's I wonder?

Only drawback is the board is rather large for a little preamp. It would be great if a PCB layout master could put out a design for this with through holes in a small footprint. Those are amazingly low distortion figures.

Last edited by xrk971; 8th September 2016 at 02:38 PM.
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Old 8th September 2016, 02:36 PM   #130
xrk971 is offline xrk971  United States
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Quote:
Originally Posted by voltwide View Post
No.
Open loop gain is a matter of voltage gain, not current gain.
Understood - this was already pointed out several posts above.
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