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Old 4th April 2008, 11:32 AM   #1641
FaTTy is offline FaTTy  Russian Federation
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JPeitzman
I do not have extensive experience of application of various high-quality resistors in audio devices. But I have constructed some variants of this amplifier with use of various parts. With caddocks my amplifier sounds very purely and openly. As a matter of fact these resistors the best that I could get for that moment so i used them and not be dissapointed all the ways
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Old 4th April 2008, 03:28 PM   #1642
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FaTTy
Thanks, that is what I seem to hear about all the Caddocks, open, pure, and detailed. I might have to give some of these resistors a shot sometime, maybe when I rebuild my JLH.
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Old 5th April 2008, 03:08 AM   #1643
Fanuc is offline Fanuc  United Kingdom
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Quote:
Originally posted by john_ellis
HI Marco

I don't know about the Toshiba 2N3055's. I suspect that they would be something like the early epi types with a relatively low Ic(max) at 60V. If you are running your JLH at +/-25V it MIGHT be that your 3055's are near the limit.

Toshiba devices appear to be quite widely "cloned", unfortunately. I may be wrong, but I think Toshiba gave up making 3055's some time ago as they have concentrated on the new fast high power devices like 2SA1302 (MJL1302 from ON semi etc).

James- yes, I've a MJL3281 version of the JLH. It's stable with a 47 pF capacitor across the feedback resistor and a 47 pF capacitor on the input base to ground. I think your line up is fine (BC560 2N3019). Many people overlook the importance of the input capacitor. It keeps the frequency response of the input stage high even if there is a load in series (external due to preamp output impedance etc).

I haven't had time to listen to it, yet! But the main difference is that on the bench, positive slew is tremendously fast compared with RCA 3055's. Those oldies could not drive even at 20 kHz (in simulation). Perhaps this is why some people can hear differences. The old RCA transistor had an fhfe of 10 kHz (min.) I typically measured 15 kHz. THe JLH amp original circuit did not have enough spare drive current to compensate for this for higher powers, but to be fair to JLH, he specified MJ481's I think, which are 4 MHz devices.

SO I have every expectation that the MJL3281 version will sound superb. When I get to listen!

Marco - I think that you may be near the limit of SOA for ordinary 2N3055's. You may like to try ON Semi 2N3055's in case their improved SOA helps. I suspect your amp may be oscillating too - this could reverse bias the base junction without you noticing (without a scope). You could try a 47 pF capacitor across the feedback resistor too even for "slow" 3055's. If the DC level changes it might indicate that that was a problem. Sometimes poor wiring can lead to oscillation (stray inductances, etc).

The simulated distortion for the protected JLH was the same as the unprotected one.

cheers
John
Hello John,

A few questions for you on your JLH design.

Have you tried a CCS load for the phase splitter with 2SC3281 outputs? Have heard this arrangement can work but mostly with a bootstrap load.

Does the 47pF caps you mention over the feedback resistor reduce the OLG bandwidth under 100Khz ? I was also wondering if creating a lag over the base of the phase splitter would get round this. ?

I have heard of one person use even faster Sanken RET transistors with Ft/GBP nearly double the 2SC3281 but large cap 80pF I think.

On a another note. Emitter degeneration. Being a singleton input without any way to cancel the second harmonic like a normal ltp can. One option is to introduce a degen resistor like 100r/200r to maybe cut it by a tenth. A difficulty for me is the current through the feedback resistors etc.

I can double the CCS current to the emitter but the feedback resistor has me stumped. Could be much more than 100uA's. Not sure on that one.

I think i will start off with MJ21194 NPNs with 10r base stoppers and a very tight layout before trying the compensation/faster stuff.

Help appreciatted

Kevin
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Old 5th April 2008, 10:39 PM   #1644
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Hi Kevin

The input stage of JLH uses a low value of emitter resistor already!
I tend to prefer 100 ohm decoupling resistor and 1k feedback instead of 220 ohm and 2.2 k (or was it 2.7k JLH used?)

This resistor limits the OLG and so using 100 ohms instead of 220 gives a useful increase in gain and approx. halves the distortion.

47 pF across 1kohm does not affect performance at 100kHz much (3dB frequency point is 300kHz).

I don't think you would need base resistors with MJ21194's. I think a small feedback capacitor is needed. The original JLH shows a peak in response around a few MHz without any feedback capacitor, and this may make the design sensitive to layout.

cheers
John
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Old 10th April 2008, 03:52 PM   #1645
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Maybe someone has and I am just bad at searching, but has anyone built the JLH with a JFET input as detailed in the original article? Seems like with a new Toshiba or something of the like it could be an interesting variation.
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Old 10th April 2008, 05:54 PM   #1646
Geoff is offline Geoff  United Kingdom
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Quote:
Originally posted by JPeitzman
Maybe someone has and I am just bad at searching, but has anyone built the JLH with a JFET input as detailed in the original article? Seems like with a new Toshiba or something of the like it could be an interesting variation.
I'm not surprised your search proved fruitless. Not one of the hundreds of JLH constructors who have contacted me has even mentioned this add-on.

This is perhaps understandable given the comment in the original article, quote:

"If the output impedance of the pre-amplifier is more than a few thousand ohms, the input stage of the amplifier should be modified to include a simple f.e.t. source follower circuit, as shown in Fig. 8. This increases the harmonic distortion to about 0.12%, and is therefore (theoretically) a less attractive solution than a better pre-amplifier."

Now if Tr4 were to be replaced with a JFet it could be a different matter

Geoff
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Old 10th April 2008, 06:11 PM   #1647
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True I did read that too in the original article, though is this the figure with the give FET? I am sure that the increase in distortion would be a different matter with modern JFET's, but I may be mistaken, it wouldn't be the first time .

You did bring up a point that I had considered too, the replacement of tr4 with a JFET, has this been tried. I have seen the PLH, interesting but not quite what I had in mind. I like the idea of a JLH with a high input impedance and some of that JFET character, but still a JLH Class-A through and through.

BTW has anyone tried Zetex transistors in their JLH. I'm doing some rebuilding and modifying and was considering trying these in addition to some Toshiba's and some various others. I am going to be ordering some for some N. Pass projects and thought I might give them a shot in my transistor rolling quest...can you tell I have tubes in my blood

Cheers!

James
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Old 10th April 2008, 11:01 PM   #1648
EUVL is offline EUVL  Europe
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For your interest, I have built, in addition to the bipolar transistor version of the JLH, a version of a bridged JLH with all FETs and crossed feedback as in Nelson Pass's XA amplifiers :

http://www.diyaudio.com/forums/showt...428#post722428

The latest version, which is my current reference, has JFET input diff pair (2SJ109BL) with 910R drain resistors. It is easy enough to re-engine this back to the single-ended version of the updated JLH as described in Geoff's excellent website, using JFETs for the first stage and the phase splitter (including the current sources), and MOSFETs for the output stage.


Patrick
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Old 12th April 2008, 04:34 PM   #1649
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Default update on the help cry

Quote:
Marco - I think that you may be near the limit of SOA for ordinary 2N3055's. You may like to try ON Semi 2N3055's in case their improved SOA helps. I suspect your amp may be oscillating too - this could reverse bias the base junction without you noticing (without a scope). You could try a 47 pF capacitor across the feedback resistor too even for "slow" 3055's. If the DC level changes it might indicate that that was a problem. Sometimes poor wiring can lead to oscillation (stray inductances, etc).
I think you are right about being near the SOA for the 2N3055. As my cap multiplier on the right hand channel was broken it was running on + and - 26 Volts and even higher on start-up. (the rails sag a little on full load).

I've added a 47 pF across the feedback resistor. This gave very little difference on DC offset, so I think it wasn't oscillating after all.

I repaired my cap. multiplier and I'm on + and - 23.5 Volts rails with a small Ig of 0.75 A. I expect them to sag a little when I increase the Iq.

Now I am doubting how to continue. I happen to have 4 spare 2N3055, so I can double output transistors, increase Iq (rails sag a little more) and hope that I'm safe (else I blow 4 trannies in one go!).

On the other hand I can put in the the 2sc3281 I bought for this purpose a while ago (I didn't dare to risk them before problems were solved.). But then I have to solve the oscillation problem which is a little beyond my knowledge and sets me up with a whole new set of pitfalls.

Advice is welcome!

MArco
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Old 14th April 2008, 07:52 PM   #1650
Fanuc is offline Fanuc  United Kingdom
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Quote:
Originally posted by john_ellis
Hi Kevin

The input stage of JLH uses a low value of emitter resistor already!
I tend to prefer 100 ohm decoupling resistor and 1k feedback instead of 220 ohm and 2.2 k (or was it 2.7k JLH used?)

This resistor limits the OLG and so using 100 ohms instead of 220 gives a useful increase in gain and approx. halves the distortion.

47 pF across 1kohm does not affect performance at 100kHz much (3dB frequency point is 300kHz).

I don't think you would need base resistors with MJ21194's. I think a small feedback capacitor is needed. The original JLH shows a peak in response around a few MHz without any feedback capacitor, and this may make the design sensitive to layout.

cheers
John
Thanks for the reply and informative answer John.

So would you say that the degen resistors of the CE transistor could be biased by a negative potential with a split rail supply? (not used to that being honest unless of diff. polarity!)

I thought at first that you mean't the internal Rb of the transistor in question. As you will know this affects noise.

PS. I am currently just starting my HNC in electronics.

Second Question,

I was thinking of using two different transistors for the OP stage of the JLH. I read briefly what you said on the issues of bootstrapping and the upper transistor speed etc. - positive feedback plus using a diode etc

So I was thinking of using a MJ3281A on top and a MJ21194 on the bottom. Both these are T-O3's. Would be interesting to see what happens with devices with broadly the same gains but different FT's(GBP) are mixed.

The gain linearity of the MJ21194 isn't that bad really.

To be honest I think we could exploit some affects of the output stage!.

I will be building your PLIL design very soon and testing it out on electrostats (seriously low Z at HF) and floorstanders (very low Z at LF) etc and will let you know my findings.

Will use the BC transistors etc.

Best Regards

Kevin
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