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Old 28th May 2011, 07:56 PM   #451
mikelm is offline mikelm  England
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Among the audio elits like Nelson Pass, John Curl, Erno Borbely etc it has been well understood for many years that although jfets consistantly give higher distortion they generally sound better. I think we just have to tweek this design to make it sing.

Personally I think it is a pity we moved away from DC linking, for me it just sounds much better across the whole frequency spectrum, generally cleaner and also better defined, tighter bass - perhaps we are throwing out the baby with the bathwater by playing conventional & safe. ? ? ?

The problems as I understand them of DC linking are:

1) DC offset - TimA, on the JLH simple class A thread has already solved this problem - I'm sure his idea can be adapted to this design.

2) Protecting a Jfet - well, after trying many fairly exotic film i/p caps, my conclusion was that "the only good cap is no cap" - so I'm prepared to take the risk of a "DC event". I'll also lower the input impedance to no more than 10K ohms and possibly a lot less - this should help a bit in this regard.

3) Worry about damaging the loudspeaker. For this I will use a small length of constantan wire soldered between 2 pins as a 0.1 ohm o/p fuse. This wire sounds as good as any o/p resistor I ever tried and much better than most and is very cheap.

If I missed anything that anyone thinks is a deal breaker here - please let me know but for for now at least there is no contest, I'm going for a DC linked version !

After pouring over hundreds of simulations and hearing Swordfishy's feedback, here is the circuit that will be my starting point - apart from the BJT VAS it's almost identical to Lineup's design on page 3. For me, the simplicity elegance of this design is too compelling to ignore.

I am also planning to experiment with combinations of fet VAS, CCS i/p load resister and i/p stage Hawksford cascode and if absolutely necessary a servo.

Oh and by the way, I would not expect the benefits of the Hawksford cascode - if there are any - to be apparent in spice. My use of this would be aiming at achieving constant power across the i/p device at all times so it may have to be used in conjunction with a CCS.

I can already hear some people saying - oh but that is getting so complicated - but for me circuits should be as simple as possible but complicated enough to sound great . . . and there's no harm in trying various things to see how they sound
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Last edited by mikelm; 28th May 2011 at 08:04 PM.
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Old 28th May 2011, 08:54 PM   #452
MiiB is offline MiiB  Denmark
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MikeLm.. could you possibly link to the JLh simple class A thread where the offset problems have been solved...

On simulation the higher order distortion drops by a magnitude if you use hawksford Cascode..and you also gain som better phasemarigen and additional gain so it should be easier to keep from occilation...In my simulation i have 3 green leds to set the Casodevoltage from the buttom for the Jfet...

Would like the DC coupled version.. but to be honest the lower DC gain by having a cap in series with tha feedback gnd resistor makes DC-triiming quite easy and not so prone to drifiting...
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Old 28th May 2011, 09:27 PM   #453
lineup is offline lineup  Sweden
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mikelm

I am glad you will test the DC-version.
This was my first idea and is surely the most elegant.
Hope it works.

I see you use a special compensation. They say it is better.
You use rather heavy degeneration of VAS transistor.
Might contribute to stability of DC-offset.

C5.
I actually simulated testing 10u 4.7u 2.2u 1uF and no cap in that position.
2.2uF was the winner in low THD at 1kHz
So, 1uF is close. Can probably vary because of the size of R10.

The gain 750/47+1 gives 17
For 1.41Vp input we need x14 to get 20Vp (25 Watt)
680/47+1 gives x15.4
... so this I suggest


I tried simulate with 2 RED LED and 1mA Hawksford cascode.
Did not show any improvement. In fact THD became a bit higher.


If you use one MOSFET for VAS, you can improve by replacing R13 potentiometer
with one 2SK170 coupled as variable current source.
MOSFET has several Volts VGS and so one can fit in JFET there.

See diagram.
25 Watt with low distortion.
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Old 28th May 2011, 10:10 PM   #454
mikelm is offline mikelm  England
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Quote:
Originally Posted by MiiB View Post
mikelm.. could you possibly link to the JLh simple class A thread where the offset problems have been solved...
here is a link to the class A amplifier web site

The Class-A Amplifier Site - JLH Class-A Update

Read the text below the second diagram.

here is a small extract . . .

"Tim has been able to achieve an output dc offset variation between switch-on and normal operating temperature of less than 50mV."

This will take a few variations to the current diagram but I think it is definitely do-able.

I won't experiment very much in spice before I build because there will be no drift to assess

mike
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Old 28th May 2011, 10:24 PM   #455
mikelm is offline mikelm  England
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Quote:
Originally Posted by lineup View Post
mikelm

I see you use a special compensation. They say it is better.
You use rather heavy degeneration of VAS transistor.
Might contribute to stability of DC-offset.
the extra degeneration decreases distortion but does not necessarily decrease OLG - it's a complex system but all in all it seems to give benefits in all significant areas

Quote:
C5.
I actually simulated testing 10u 4.7u 2.2u 1uF and no cap in that position.
2.2uF was the winner in low THD at 1kHz
thx - I'll try 2.2uF also

Quote:
The gain 750/47+1 gives 17
For 1.41Vp input we need x14 to get 20Vp (25 Watt)
680/47+1 gives x15.4
... so this I suggest
Yes - but DVD's seems to recorded at a lower level so I may even need to go higher

Quote:
I tried simulate with 2 RED LED and 1mA Hawksford cascode.
Did not show any improvement. In fact THD became a bit higher.
I'll check the Higher order levels but I also found stability was improved but ultimately I'll let my ears be the judge

Quote:
If you use one MOSFET for VAS, you can improve by replacing R13 potentiometer
with one 2SK170 coupled as variable current source.
MOSFET has several Volts VGS and so one can fit in JFET there.

See diagram.
25 Watt with low distortion.
Thx - I have tried this and I think I could fit in also with a BJT with 200 ohm degen res but I need to check

Also just wanted to say thanks very much for such a great little circuit idea I can't wait to build it.

mike

Last edited by mikelm; 28th May 2011 at 10:30 PM.
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Old 28th May 2011, 10:25 PM   #456
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by MiiB View Post
MikeLm.. could you possibly link to the JLh simple class A thread where the offset problems have been solved...

On simulation the higher order distortion drops by a magnitude if you use hawksford Cascode..and you also gain som better phasemarigen and additional gain so it should be easier to keep from occilation...In my simulation i have 3 green leds to set the Casodevoltage from the buttom for the Jfet...

Would like the DC coupled version.. but to be honest the lower DC gain by having a cap in series with tha feedback gnd resistor makes DC-triiming quite easy and not so prone to drifiting...
Actually MiiB is right.

The Hawksford Cascode does lower the distortion as output admitanced goes up because of the lack of BE modulation.

But implemented the wrong way it could make the amp prone to oscillation. Reallife inductance can play with you.
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Old 29th May 2011, 12:29 AM   #457
mikelm is offline mikelm  England
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Default perhaps 2 birds with one stone ?

This measure lineup suggested may help a lot with DC offset on a DC linked version and inject a bit more life into the amp with a jfet input.

It adds about 6db of OLG just as Hugh predicted giving about 77db OLG from DC to about 1Khz

The compensation measures are not optional with this design but with the 1K 1nF i/p filter you will hardly see the difference.

The 200 ohm VAS degeneration resistor may also be compulsory but I did not check.

Would be great if swordfishy feels like checking this

An output snubber of 4ohm & 100n allows the compensation cap to be smaller perhaps 200pF rather than 330pF
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Old 29th May 2011, 03:22 AM   #458
AKSA is offline AKSA  Australia
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Rudi,

My experiments with cascodes were performed some years back on power amps using LTP input stages and before I began to use LTSpice. I did not notice oscillation at all, but there was no discernible improvement in the sound that I noticed. I concluded that cascodes should be used only where the voltage rating of the input device, such as a jfet, was not adequate for the job, and that the best of them was the Hawksford cascode. For my personal preference for the lowest possible component count, I came to the conclusion that they did nothing of use to the signal path, but this was my experience, and clearly is not yours.

I did use a cascode in a MC headamp I designed, the Paris. But it is a partial Hawksford cascode (modulation with 50% of signal, not 100%), and designed for low noise and a particular distortion profile.

Mike, 200R of degeneration seems a lot. If the VAS runs at 11mA, then re is 26/11 = 2.3R. In my experience, ten times this resistance, near on 22R, is sufficient to swamp the non-linearities of this intrinsic emitter resistance, and further degeneration creates too much current variation in the first stage, detracting from its linearity due to Vbe/Vgs changes on the input device.

For some reason a number of posts were silently removed without trace in the last 12 hours. I do not know why this happened, but I would hope there is a reason for it.

Like the direction of this circuit, it's very good....

Hugh
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Old 29th May 2011, 05:09 AM   #459
sonnya is offline sonnya  Denmark
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As hugh , i would like to it now also. If there is a good reason i think we should know it.! (

- Sonny

Ps: ask the founder why he does not answer my email!?
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Old 29th May 2011, 05:26 AM   #460
GregH2 is offline GregH2  Australia
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All,

Again, I have told a fib. I think, theoretically, a small feedback cap should improve bass by reducing input stage degeneration at low frequencies and increasing OLG. So my idea posted at the early hours of this morning do not apply.

I have spent all morning experimenting and believe I have cracked this amplifier. I have tried, to name a few things:

Different sizes of feedback capacitors, including NO capacitor at all,
Different currents in the VAS and input stage,
High Degeneraton and low degeneration in both the input and VAS stages.
I have tried mosfets, JFETs and now even a BJT in the input stage.
I have tried different types of feedback networks.
I have dealt with DC offset issues, almost incurable oscillation and all kinds of compensation.

I have dealt with all of these simultaneously with different devices. Now I think I know how to make this amplifier sound good without resorting to further circuitry.

The rule is, use high gain devices then degenerate to get it stable.

The amp appreciates a high impedance feedback network, i am using 1k/100r. I think this due to the fairly high positive voltage required to bias the input with such a feedback network. I don't know why, but the bass performance is better this way. Maybe hugh can explain. This is what I first used with the MOSFET design that I liked so much, and now also with the bjt design I am currently listening to. Unfortunately due to the relatively high degeneration this creates, a jfet is no good here. You need to use a MOSFET or bjt for the input stage. The bc550 verges on clinical, although the bass is excellent. I suggest the MOSFET for a bit of warmth.

I tried to get the jfet to work, I really did, but I think without a cascode or whatever, it just doesn't cut it. I tried reducing the input and vas degeneration to get a bit more olg, but I just ended up with oscillation issues. A miller cap didn't help the sound at all.

Without resorting to additional active devices, I think I have found the best design for this amplifier, and really, it sounds excellent. I will post two schematics soon.

Last edited by GregH2; 29th May 2011 at 05:28 AM.
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