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The Frugalamp by OS
The Frugalamp by OS
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Old 20th November 2008, 12:18 PM   #101
lineup is offline lineup  Sweden
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Default Re: Frugalamp

Quote:
Originally posted by sandyK
lineup
It's not feasible at these much higher rail voltages, but some years ago, with a phono preamp using an SSM2220 LTP,
and +-15V rails , I achieved a very good null of the offset, simply by connecting a 20M 1% resistor between collector and base of the input device. The directly coupled MM cartridge then had virtually no DC through it.

SandyK
Yes, you are right.
Using big value resistors, eventually with some 0.7 V diodes to create some lower voltage reference, is many times a good enough solution.
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Old 20th November 2008, 12:24 PM   #102
Symon is offline Symon  United Kingdom
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Quote:
Originally posted by ostripper
Could also CM the VAS instead (make a big symasym)...

your VAS already has a simple CM, I assume you mean replace the diode with a transistor wired as a diode
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Old 20th November 2008, 06:42 PM   #103
ostripper is offline ostripper  United States
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Quote:
The VAS doesn't leave class A, as it doesn't switch off. Finding the current that it works most efficiently at is the key. You will cut distortion by a good margin by lowering that 14mA to ~8mA.
I have taken your recommendations in 2 areas ,lowered my
VAS to 10mA (my VAS did get a little toasty when cranked)
and went back to 1.7mA for input section.

Also, I tried the 2sa1381/2sc3503 combo , and it "likes" 10ma
also, same package, better specs. (found them in crt stuff)
Frugal 1 is finalized, except maybe EF VAS.

Frugal 2 is where I want to go with the symmetrical VAS
(ampslab type Bi-120).

Quote:
By AKSA -Yes, you could, and if you do, you have replicated the Hitachi Application Note from 1972 which was developed for their new mosfets, and which is used in many Class AB Pro-audio amps because it is rugged, dependable, and inexpensive
Yes, that is where I think ampslab "ported" that design from.
PS... new update to frugal 2 soon.
OS
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Old 20th November 2008, 07:22 PM   #104
MJL21193 is offline MJL21193  Canada
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Quote:
Originally posted by ostripper


I have taken your recommendations in 2 areas ,lowered my
VAS to 10mA (my VAS did get a little toasty when cranked)
and went back to 1.7mA for input section.

Hi OS,
I didn't say anything about the input section, just that there was a 1 watt resistor there and that the way you had the 3rd transistor on the CM configured didn't work for me in simulation.
Running the differential pair at a higher current (within limits) seems to be beneficial.

Using the 2SA1381 for VAS, you can get away without the EF. Simulation says it is much improved.
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Old 20th November 2008, 07:28 PM   #105
andy_c is offline andy_c  United States
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Quote:
Originally posted by ostripper
Also, I tried the 2sa1381/2sc3503 combo , and it "likes" 10ma
also, same package, better specs. (found them in crt stuff)
Frugal 1 is finalized, except maybe EF VAS.
Also, if you need to buy some new parts, Mouser charges less for the KSA1381/KSC3503 than they do for the MJE340/MJE350.
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Old 20th November 2008, 09:22 PM   #106
GK is offline GK  Australia
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Quote:
Originally posted by Symon
Just an observation, but the voltage gain of the inverting side of the input pair is very low compared with the non-inverting side. because the dynamic impedance of the current mirror input is low, compared to the output impedance.

It will work, but you aren't really driving the VAS with equal / balanced signals.

Thatís exactly what I said; the reason why I used a current mirror tail for the VAS so as to avoid modulation of the tail current.

With that matter addressed, with regards to driving the differential VAS with unequal signals it really doesn't matters as a differential amplifier is only interested in the difference between its inputs. Distortion is still lower (over a "balanced" resistive load arrangement) as the CM loaded LTP gain is much higher and the LTP doesn't work nearly as hard.

Cheers,
Glen
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Old 20th November 2008, 09:25 PM   #107
GK is offline GK  Australia
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Quote:
Originally posted by AKSA
However, this will alter OLG of the LTP, in turn changing compensation by reducing fb factor. You will need to reduce the lag compensation to accommodate this change.

Completely wrong. You will only need to alter the compensation (WRT LTP gain) if the gm of the LTP is altered.

Cheers,
Glen
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Old 20th November 2008, 10:10 PM   #108
ostripper is offline ostripper  United States
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Quote:
By andy c. - Also, if you need to buy some new parts, Mouser charges less for the KSA1381/KSC3503 than they do for the MJE340/MJE350
Mouser is a good company, free air at ground rates for new customers . Good prices too, that's how I built these for 21$
apiece.


Quote:
by Symon - your VAS already has a simple CM, I assume you mean replace the diode with a transistor wired as a diode
you guessed right..next post..
OS
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Old 20th November 2008, 11:23 PM   #109
vynuhl.addict is offline vynuhl.addict  Canada
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Quote:
Originally posted by G.Kleinschmidt



Completely wrong. You will only need to alter the compensation (WRT LTP gain) if the gm of the LTP is altered.

Cheers,
Glen

Glen,

This is right, remember Aksa noted that after removing the Current Mirror the gain in the LTP will decrease now that it is R loaded, with lower fb factor the lag does not need to remain as large as the OLG as a whole has now decreased. If it were the opposite, replacing passive with a CM, then you would increase lag comp. The ever lasting balance between FB factor and the need to lower the dominant pole as olg is increased, you know this very well..




Colin
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Old 20th November 2008, 11:58 PM   #110
ostripper is offline ostripper  United States
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Here it is.. I want it.

Click the image to open in full size.

This is what I want for a amp,they say this is tricky to build,
but it should sound golden. After a lot of research , it seems
if the bandwidth/gain of the 2 sides of the VAS are not equal a "pole-zero
doublet" will be formed. This must be why one must compensate
at the 'backside" of the VAS. I know it has been done,
in fact , some very good amps have a similar topology.So, I'll give it a shot,best way to learn, build it..

I've yet to choose my miller comp.

OS
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