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Old 6th September 2004, 09:08 PM   #21
andy_c is offline andy_c  United States
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Quote:
Originally posted by Geoff
Is it safe to assume that the current anomalies are generated by inaccurate models and that reality is closer to the simulation results obtained using Andy's revised model?
I wish I knew the answer to that . One thing that makes me suspicious is Figures 4 and 6 in the MJL3281A data sheet. The equations SPICE uses with the Early Voltage show that Figure 4 should be shifted upward from Figure 6 (beta increases as Vce increases). But there's so little change it makes me wonder whether this is a measurement error or the devices just don't fit the theoretical equations that well. Another technique for finding the Early Voltage is to look at Figure 8 and see where the linear part of the curves intersect the negative Vce axis. It's clear that they don't intersect at the same point, yet the Early Voltage theory says they should. In the end, I just guesstimated how much Figure 4 might be shifted upward relative to Figure 6 and came up with the 200 Volt number for VAF. I don't know if this really reflects reality. If figures 4 and 6 really reflect what the devices do at Vce = 5V and Vce = 20V, then the model should reflect operation in the circuit pretty well.
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Old 7th September 2004, 06:12 AM   #22
Ejam is online now Ejam  Australia
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Default Using some of the 25W-8 features

Graham

Thanks for the reply. Two quick questions about using some of the features of your amp on the JLH amp. Talking about the JLH 1969 version modified for +/- power supplies. I think this is known as the Evolution modification as per Geoff's website.

First, could you use the 390 ohm emitter and 390 ohm collector loads for the driver transistor (2SC3421) with Rb = 270 ohms. Second, could you use the Rz modification, assuming the 2k7 and 220 feedback divider for DC offset stabilisation. Is feedback compensation needed here, you use 22pF but for this amp maybe 10pF? Output stage using the 2SC5200 and input stage using the 2SA970.

Anyone

If you could stimulate these mods, nice to know how they go. I suppose I like the idea of doing it with only four transistors...

Regards
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Old 8th September 2004, 07:43 AM   #23
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Hi Andy,

Its great that you have managed to get 'Early Voltage' for Spice with On-Semi devices behaving more alike those of real life, and thus clarified Geoff's findings.
JCX's MJL3281A model must have had an EFV of around 20V for a 200V rated device; this does not make sense. Maybe there are other simulated amplifier designs that are not getting a second look because they too appear to behave worse than they actually do in real life.

Hi Ejam,

My amplifier has a differential input stage which immediately makes it a different beastie to JLH designs.
The original JLH circuit is basically stable and as such did not need feedback compensation if sensibly constructed and used, but when modern high frequency transistors it might become necessary, with an output Zobel too. ( I use 47n and 2.2 ohms for mine where 100n and 10 ohms has been common.)

To use 390 ohm output stage resistors the driver needs to have a higher level of linear current drive from the input transistor, also the four transistor design will always send a current wave through the loudspeaker as capacitors charge at start-up unless the bias is 'slugged'.

You mention the 2k7/220 ohm feedback resistors. These work fine but being in series with the first transistor emitter they degenerate first stage gain quite considerably.

'Doing it with four' is a decent challenge though, and thus I have enclosed this simulated circuit; i.e. not real-world tested. It does spec measure better than the original, but as in the old 'swings and roundabouts' saying, it might not actually sound better than those that have been hands+ears-on optimised via considerable user communication through Geoff's site.

Cheers ....... Graham.
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Old 8th September 2004, 01:54 PM   #24
Ejam is online now Ejam  Australia
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Default GM mod to the JLH Amp

Graham

Thanks for your efforts in "doing it" with only four transistors. The JLH reminds me of a Dyanco Stereo 70, input, concertina phase splitter and pp output stage. If it can be done with valves, why not with transistors!!

I must confess that there are a few oddities with the circuit. For example, the DC path of the feedback is linked to both the input transistor via the 680 ohm resistor and to the junction of the 390 ohm and 270 ohm of the bootstrap. There is a 1000uF cap across the pot and a 22pF compensation cap across the lot. Is the purpose of this arrangement to "slug" the bias on start up by feedback?

Also shouldn't the collector load of the input transistor (680 ohm) be to the negative rail rather than across the base - emitter of the driver transistor?
Would not the presented arrangement unbalance the driver transistor?

What happened to Rz and the RC zobel network?

Lastly I thought that the Gain Bandwidth Product of the input stage was fixed. Hence a lower gain results in a greater bandwidth. Given that the JLH is a low feedback amplifier, then less gain is required by all stages. Surely three stage of low gain, wide bandwidth and low feedback is preferable to three high gain, low bandwidth and high feedback. Regarding linear current driver, why not use an inherent linear high current transistor such as the 2SC3421.

Regards

Ejam
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Old 8th September 2004, 09:07 PM   #25
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Hi Ejam.

I did as you asked, and get a load of questions.

Most four transistor JLH circuits do not need Zobels, I cannot know whether this one does until it is constructed.

The pot IS R.z and the 100uF is part of it. You can't load the output transistor base of the four transistor circuit.

Why does the first collector load need to go to ground, and how will this imballance driver input , this has high impedance for first collector and thus good first stage gain.

First stage gain and speed is limited by the high value 220 ohm emitter resistor in the original, hence my use of the lower value.

Remember low feedback means low damping, my circuit is better for complex loudspeakers.

I DO USE A 2SC3421 ?

Cheers .......... Graham.
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Old 8th September 2004, 11:11 PM   #26
Ejam is online now Ejam  Australia
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Default No more questions but a few observations

Graham

As a novice but not a complete novice, the purpose of your feedback network was not obvious. Having stated in your earlier post that turn on was a problem for the JLH then I assumed that linking the network back into the bootstrap circuit which in turn is linked back to the positive rail was to avoid the turn on problems.

Second you use 10k as the Rz which is significantly different from the value of 100k to 1M that your amp uses. I think it is fair enough to ask where was R To me the 10k pot was adjusting the feedback.

You mentioned that the BC556B collector was to ground, where I asked why not to the negative rail.

Refering to the imbalance, if current is going thru BC556B then it flows through the 680 ohm resistor then thru the 390 ohm resistor where it joins the current flowing thru the 2SC3421. Hence you have more current thru the bottom 390 ohm resistor then you have thru the top 390 ohm resistor, hence an imbalance. Or does the 680 ohm resistor act as a bootstrap to the 2SC3421 to give a high input impedance to the 2SC3421? Normally bootstrap resistors have significantly higher resistance than 680 ohms.

You mentioned a 220 ohm resistor, where is that? Your modified JLH does not have a 220 ohm resistor.

I must apologise for writing "driver" instead of "drive" for the 2SC3421. I was refering to using the 2SC3421 for the input transistor rather than the BC556B.

I gather you are getting a little testie with my questions so I shalln't ask any more. Thanks very much for your help.

Ejam
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Old 9th September 2004, 01:16 AM   #27
andy_c is offline andy_c  United States
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Quote:
Originally posted by Graham Maynard
JCX's MJL3281A model must have had an EFV of around 20V for a 200V rated device; this does not make sense. Maybe there are other simulated amplifier designs that are not getting a second look because they too appear to behave worse than they actually do in real life.
That model, which is from the OnSemi web site, has an Early Voltage VAF of 46.776V. Looking at the characteristic curves of the device in Figure 8 of its data sheet, it's seen for the lowest Ib (0.5A) that:

Vce1 = 10V
Ic1 = 22A

Vce2 = 5V
Ic2 = 19A

Finding the intersection of this line with the negative Vce axis gives Vce = -VA = -36.7V, which is even lower than the 46.77V value in the OnSemi models. Yet if you simulate beta vs Ic at various Vce values with that Early Voltage value, the simulation would show that theres a significant increase of beta as Vce goes from 5V to 20V. But the data sheet shows that they are nearly identical (Figures 4 and 6 in the data sheet). For the data sheet characteristic curves (Figure 8), the lowest Ib is 0.5A, much greater than what's needed to compute the beta vs Ic curves, My only guess is that when the base current is small, the Ic vs Vce curves at a fixed Ib must intersect the negative Vce axis at a much more negative voltage value than they do at higher Ib values. So apparently the device doesn't conform well to the Early Voltage theory, which says they should all intersect the Vce axis at the same point.
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Old 9th September 2004, 06:54 AM   #28
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Hi Ejam,

I am sorry, but you are piling on even more questions before you are thinking it out for yourself.

You think you are finding fault, but sadly you are wrong. Either build what I have suggested or simulate it so that you can come to understan the design.

The only problem I would be wary of is power supply and temperature change causing output potential drift, but that is what you get with a single input transistor, and why I changed to differential input which holds the output very accurately.

Cheers ............ Graham.
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Old 9th September 2004, 09:03 AM   #29
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Hi Andy,

I have not taken a great deal of interest in Spice because I prefer to try circuits in real life. It is though an excellent comparative guide. Your figures are very interesting also.

It has been my past observation that JLH based amplifier circuits perform better in real life than Spice would suggest.

The Forward Early Voltage specification of models could have a bearing here, though as I have never studied Spice itself I did not know that it had the abreviation VAF. My simulator does not use these same abbreviations.

When I ran a few examinations I noted that the Reverse Early Voltage figure also affected the JLH output simulation. Is this abbreviated as VAR ? If yes, are the (different) figures quoted above in this string correct for a good modern 200V semiconductor ?

I mention this because a VAF of 46.776V on my simulator would be above the figure where the double humping effect becomes apparent with +/-20V output through 8 ohms from +/-25V rails.

Over to you, for I do not have experience here.

Cheers ............. Graham.
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Old 9th September 2004, 10:04 PM   #30
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Default Re: Scaling Power Output to 10 - 12W

Quote:
Originally posted by Ejam
Graham

I read your article and like the amp. I am from the valve school having build many amps but must confess ignorance when it comes to transistors. I would like to build a 10 - 12W version of your amp similar to the original JLH version. I have very efficient speakers and a small room and no need for the heat of the 25W version. My 8W 300B amp is more than enough. I would think that the rails of +/- 15V would be in the ball park. Can Tr5 be omitted if the amp rarely gets to maximum output ? Hope you can help with some suggestions.

Regards
If you have good eficient speakers needing 15W, instead of a class A SS amp, I would build a tube amp, since you have much experience with valves. The sound quality will be much better.


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