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Maybe it's time I joined the party too. I probably have enough bits lying around to make up a pair of mono-blocks. I have two EI transformers left over from another project, each one puts out 24Va.c. at 5.5Amps.
I have 4 heatsinks (see photo attached showing one of them, they are all the same) that were stripped out of old regulated power supplies, I will add additional metal heatsinking to improve power dissipation. With 4 in total I can use 2 of these heatsinks per channel. As you can see from the photo, there are 4 power devices riveted onto each heatsink which if they are all still working are already matched as they are wired up in parallel on the flip side of the heatsink with 0R1 emitter resistors. So I'd end up with quadruple-2N3055 outputs.
I read on this thread and others that the 2N3055 is worse than rubbish. One of the main complaints is bandwidth - although this does apparently help with stability. Another major complaint is beta-droop. I checked the data sheet (see attached photo) and it looks pretty awful. However, there is a current range where Hfe doesn't vary too much - looks to me like you want to avoid going above 500mA. So with 4 devices biassed at 250mA each I'd have an idle current of 1A and a peak output of 2A as well as very hot heatsinks - but it looks doable.
Would I be wasting two good transformers on these ancient transistors ? - did anyone try a pair of BD139s as drivers for the output devices ?
I have 4 heatsinks (see photo attached showing one of them, they are all the same) that were stripped out of old regulated power supplies, I will add additional metal heatsinking to improve power dissipation. With 4 in total I can use 2 of these heatsinks per channel. As you can see from the photo, there are 4 power devices riveted onto each heatsink which if they are all still working are already matched as they are wired up in parallel on the flip side of the heatsink with 0R1 emitter resistors. So I'd end up with quadruple-2N3055 outputs.
I read on this thread and others that the 2N3055 is worse than rubbish. One of the main complaints is bandwidth - although this does apparently help with stability. Another major complaint is beta-droop. I checked the data sheet (see attached photo) and it looks pretty awful. However, there is a current range where Hfe doesn't vary too much - looks to me like you want to avoid going above 500mA. So with 4 devices biassed at 250mA each I'd have an idle current of 1A and a peak output of 2A as well as very hot heatsinks - but it looks doable.
Would I be wasting two good transformers on these ancient transistors ? - did anyone try a pair of BD139s as drivers for the output devices ?
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The MJ15003 are a good choice, both sound-wise and spec-wise. Similar to the 21193 and a little cheaper. I also use 139s as drivers with no complaints.
I used to use 2N3055s as outputs (and in the regulators where I still use them). This is a good sounding amp with the 3055s but the 15003s take it to the next level of smoothness. In other words the 3055s sound a little grainy by comparison.
I used to use 2N3055s as outputs (and in the regulators where I still use them). This is a good sounding amp with the 3055s but the 15003s take it to the next level of smoothness. In other words the 3055s sound a little grainy by comparison.
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I think it would easiest for me to use what I have of course, which means starting of with the 2N3055H's. If I then wanted to swap them out I'd want to replace them with TO-3s so as to use the same heatsinks. I usually buy from digikey, they don't have the MJL's in that package style but they do have the MJ15003's though.
I've looked at the data sheet and it is very similar to the 2N3055, ft is only 2MHz which is actually a bit lower than the 2N3055H, suggesting Cob is similar. The beta droop is improved though (see attached) giving good performance out to 1A. But if this is the only benefit I'm already mitigating this by employing additional output pairs ??
Edit: attached beta curve for the MJL part for comparison, it's the better part on this basis.
So, if I use 2N3055 in quantity I mitigate beta droop. Does this 'fix' the problem, what else do I need to learn about transistor performance ?
I've looked at the data sheet and it is very similar to the 2N3055, ft is only 2MHz which is actually a bit lower than the 2N3055H, suggesting Cob is similar. The beta droop is improved though (see attached) giving good performance out to 1A. But if this is the only benefit I'm already mitigating this by employing additional output pairs ??
Edit: attached beta curve for the MJL part for comparison, it's the better part on this basis.
So, if I use 2N3055 in quantity I mitigate beta droop. Does this 'fix' the problem, what else do I need to learn about transistor performance ?
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The 15003 was a direct swap. I was using doubled up 2N3055s before (ie: 8 devices per stereo amp). The MJ15003 I used singularly (4 per stereo amp). It took half an hour to do the swap and measure up only to find I didn't need to adjust anything.
Have you seen Geoffs site? This is the transistor substitute page.
Have you seen Geoffs site? This is the transistor substitute page.
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Thanks, that link certainly paints a strong recommendation for the MJ15003. Trouble is, it means I'm really starting from scratch rather than using what I got, but it seems from the experiences of others that it's worth the trouble.
Still I'm surprised and intrigues why the MJ1 is providing such clear sonic benefits over the 2N3055 - in your case the beta droop should not have been a big issue as you had double the number of 2N3055's - I can't see why from the datasheets and I feel there should be some explanation.
p.s. a bit off topic - but did anyone have a good experience with 2N3055s as Followers only - i.e. not strictly the JLH ?
Still I'm surprised and intrigues why the MJ1 is providing such clear sonic benefits over the 2N3055 - in your case the beta droop should not have been a big issue as you had double the number of 2N3055's - I can't see why from the datasheets and I feel there should be some explanation.
p.s. a bit off topic - but did anyone have a good experience with 2N3055s as Followers only - i.e. not strictly the JLH ?
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Just build it with what you have and sub the other parts later when and if you decide you want more. They are a direct swap.
Also on Geoff's site is the "JLH update" - see the schematic called the "Penultimate Circuit".
I built this one a few years ago and it still remains my favourite amplifier. Highly recommended if you can be bothered to add the extra (few and cheap) parts.
The difference between the 2N3055 and MJ15003 was not, in my opinion, a night and day difference like some would seem to make out. I feel it is worth pressing ahead with the build even with the 2N3055s.
P.S. If I also remember correctly I read somewhere else that the MJL21193 issue noted in the link I gave was later found not to be such an issue. I haven't tried them myself on a JLH. I normally like the sound of the MJL21193, about 85% as much as I like the MJL3281A, generally speaking.
P.S. If I also remember correctly I read somewhere else that the MJL21193 issue noted in the link I gave was later found not to be such an issue. I haven't tried them myself on a JLH. I normally like the sound of the MJL21193, about 85% as much as I like the MJL3281A, generally speaking.
MJL is the plastic package equivalent of the metal package MJ device.
Check the datasheet. The thermal data is slightly different between a To264 MJL and a To3 MJ
Folks
There seems to be a lot of "hot air" about the 2N3055. The 2N3055 today is not the same as RCA made originally. TOday's epitaxial type will have a higher fT than the original and should give a good frequency response.
But it has still got droop. A discussion of the standing current occured in Wireless WOrld after publishing the JLLH article. Gain droop did not feature in JLLH's reply, but he came close. The point is that the standing current needs to be higher than it should be from simple calculations. Typically, if the gain droop is 55 to 45 from 1A to 2A then the quiescent current of a Class A should be nearer 1.2A than 1A. The point is that the lower output transistor can only function by stealing current from the base from the upper, so if there is insufficient, the amplifier could go into Class B at higher outputs. I don't know if this is the cause of the 2N3055 JLLH sounding "bad", but it won't help. JLLH alluded to this but did not quite say so.
The MJ21194 transistor has a much reduced droop and this distortion mechanism will be less. I have not run a side-by-side test but if modern 2N3055's are used and run with a highish bias they should be fine.
John
There seems to be a lot of "hot air" about the 2N3055. The 2N3055 today is not the same as RCA made originally. TOday's epitaxial type will have a higher fT than the original and should give a good frequency response.
But it has still got droop. A discussion of the standing current occured in Wireless WOrld after publishing the JLLH article. Gain droop did not feature in JLLH's reply, but he came close. The point is that the standing current needs to be higher than it should be from simple calculations. Typically, if the gain droop is 55 to 45 from 1A to 2A then the quiescent current of a Class A should be nearer 1.2A than 1A. The point is that the lower output transistor can only function by stealing current from the base from the upper, so if there is insufficient, the amplifier could go into Class B at higher outputs. I don't know if this is the cause of the 2N3055 JLLH sounding "bad", but it won't help. JLLH alluded to this but did not quite say so.
The MJ21194 transistor has a much reduced droop and this distortion mechanism will be less. I have not run a side-by-side test but if modern 2N3055's are used and run with a highish bias they should be fine.
John
I initially built my JLH (Geoff's 'penultimate') using old 2N3055s, but had trouble stabilizing the bias current, it kept drifting around. Upgrading to the MJ15003 solved this.
As for the sound difference, in a different amplifier (the P3A from ESP, a Class AB amp at Elliott Sound Products - The Audio Pages (Main Index)) I heard a dramatic difference when upgrading from 2N3055 to MJ15003. I never listened to the JLH with 2N3055s because of the bias adjustment problem.
As for the sound difference, in a different amplifier (the P3A from ESP, a Class AB amp at Elliott Sound Products - The Audio Pages (Main Index)) I heard a dramatic difference when upgrading from 2N3055 to MJ15003. I never listened to the JLH with 2N3055s because of the bias adjustment problem.
I can recommend JLH Update. Link: The Class-A Amplifier Site - JLH Class-A Update
It is a very good Class A amplifier.
In my SPICE I used BD139 as replacement for 2SC3421. This is possible and gives somewhat lower THD distortion.
But there is a little price to pay. The 3rd harmonic increases in spectrum and there is an overshot in the otherwise flat frequency curve.
I recommend using MJ15003. The amplifier obviously likes them.
And together with 2SC3421 it makes amplifier in nearly perfect harmony with low distortion.
And this all without any compensation capacitors.
Enjoy
It is a very good Class A amplifier.
In my SPICE I used BD139 as replacement for 2SC3421. This is possible and gives somewhat lower THD distortion.
But there is a little price to pay. The 3rd harmonic increases in spectrum and there is an overshot in the otherwise flat frequency curve.
I recommend using MJ15003. The amplifier obviously likes them.
And together with 2SC3421 it makes amplifier in nearly perfect harmony with low distortion.
And this all without any compensation capacitors.
Enjoy
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Well, my 2N3055's are the 'H' version, which I understand have ft 2.5MHz. They are 2nd hand, probably around 10 yrs old. So I don't know if these are 'new' ? Actually I haven't confirmed they work. I should get out a multi-meter sometime soon and check them [http://www.electronics-radio.com/articles/test-methods/meters/multimeter-diode-transistor-test.php]
Hi all
I have just measured the (approximate) fhfe of three 2N3055's:
RCA hometaxial (date code '75, before they were "H") 26 kHz (at 1A)
ST (epi) date code '03 85 kHz
ON Semi (epi) date code '08 52 kHz
And for good measure an ST 2N3772 (date code '0) 80 kHz (at 4A)
Therefore, the epi transistors should sound better than the old "H" which had an fT of 800kHz (min) or 200kHz (min. for the 2N3772).
It seems that my simple fhfe test circuit is suffering from residual inductance in the "non-inductive" resistor so I suspect that the 80 kHz figures are a little high.
Will do a calib. run.
I suspect that Bigun's "H" types are probably epi which have been built with a bigger version of the chip or something to ensure that they meet the original SOA rating, and probably do have higher fT's. If they are RCA they ought to be the old (real) hometaxial.
You can see why 2N3055(old) amps might not have sounded as good as the hype: especially in JLLH's 10-watter, the bootstrap drive can't work faster than the output, so the whole thing is sluggish to respond to fast audio signals. The MJ481 I think JLLH specified should have been OK (4 MHz, same as MJ21194).
John
I have just measured the (approximate) fhfe of three 2N3055's:
RCA hometaxial (date code '75, before they were "H") 26 kHz (at 1A)
ST (epi) date code '03 85 kHz
ON Semi (epi) date code '08 52 kHz
And for good measure an ST 2N3772 (date code '0) 80 kHz (at 4A)
Therefore, the epi transistors should sound better than the old "H" which had an fT of 800kHz (min) or 200kHz (min. for the 2N3772).
It seems that my simple fhfe test circuit is suffering from residual inductance in the "non-inductive" resistor so I suspect that the 80 kHz figures are a little high.
Will do a calib. run.
I suspect that Bigun's "H" types are probably epi which have been built with a bigger version of the chip or something to ensure that they meet the original SOA rating, and probably do have higher fT's. If they are RCA they ought to be the old (real) hometaxial.
You can see why 2N3055(old) amps might not have sounded as good as the hype: especially in JLLH's 10-watter, the bootstrap drive can't work faster than the output, so the whole thing is sluggish to respond to fast audio signals. The MJ481 I think JLLH specified should have been OK (4 MHz, same as MJ21194).
John
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I'm not going to use split supplies, so I will have an output condenser. This means the load current goes through the output cap and the 'last' psu cap. Even with split rails I'd have my load current going through the psu caps, both rails. But in contrast to the split rail version the single rail version has the disadvantage that the nfb network does not take off just before the load and reference to the other (gnd) side of the load - rather it takes off before the output cap and can not correct for any non-linearities this cap introduces. The split rail version is better, but I want to stick with the 1969 version and also have the benefit of speaker protection against dc.
A possible tweak to the '69 circuit would be to take a small portion of the nfb signal from after the outptut cap. How would I do this - has anybody found it worthwhile to do this ?
A related circuit is the Leak Delta 70, it has an interesting arrangement that bootstraps the input but there is a feedback resistor from the load side of the output cap - I can't quite understand what it is doing ?
You can find schematic here: http://www.diyaudio.com/forums/soli...-70-attempt-diy-overhaul-buy-another-amp.html
Can you help me understand this ?
A possible tweak to the '69 circuit would be to take a small portion of the nfb signal from after the outptut cap. How would I do this - has anybody found it worthwhile to do this ?
A related circuit is the Leak Delta 70, it has an interesting arrangement that bootstraps the input but there is a feedback resistor from the load side of the output cap - I can't quite understand what it is doing ?
You can find schematic here: http://www.diyaudio.com/forums/soli...-70-attempt-diy-overhaul-buy-another-amp.html
Can you help me understand this ?