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Old 21st December 2012, 03:00 PM   #7881
a.wayne is offline a.wayne  United States
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Yeah , but i have always noticed a tonal balance difference when paralleling pre outputs ...
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Old 21st December 2012, 03:54 PM   #7882
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Originally Posted by a.wayne View Post
Yeah , but i have always noticed a tonal balance difference when paralleling pre outputs ...
That should be true . There is a very strong argument for pre amps to be mini power amps . If the pre amp could almost drive speakers directly it is useful .

As I was saying before some power op amps look almost good enough to be universal devices . I think the market could use a proper device which is above NE5534 and would drive speakers . Dvv's boosters seem valid here .

5 is a minimum , 10 is what is taught . 3 can be OK . Less than 1 often happens between input pair and VAS . If it matters into the amp why not the VAS ? Transconductance ( 1 :1) , I don't believe in it although it seems to work with some amps . Using a high gain transistor for the VAS can do magic .

Best to have late trains than no trains . My grandfathers name was Beeching , the man who closed the railways was Dr Beeching ( cousin ) . No one liked him I'm told .
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Old 21st December 2012, 05:17 PM   #7883
dvv is offline dvv  Serbia
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It's really all about current capability. If a preamp output can deliver some serious currents (e.g 200-300 mA, mine will do better than 2 Amps), and has a reasonably low output impedance, then parallel driving should not be a problem.

Tubes are cursed here. I made three headphone amps/preamps, one uses BJTs all over, the other has FET inputs and MOSFET outputs, and the third is an all tube design, featuring no global NFB. The tube job was designed by a good friend, who has been designing tube gear for the last 25 years or so, I wouldn't trust myself with tubes to make a unity gain buffer.

While it does music very well indeed, it is EXTREMELY load sensitive. Use 30 Ohms cans and the bass almost ain't gonna be there, but use 100 Ohm cans and it really comes on song.

God forbid that you should try driving cans, even 600 Ohms cans (Beyer Dynamic) in parallel with the power amp input (Marantz 170 DC, 82 Kohms, FET input) and the overall volume droops by about a half. PSU uses full regulation, 2 50 VA toroids for full true dual mono (same as the SS units).

On the other hand, both SS units will drive 8 Ohms in parallel with 27k of the HK 6550 with aplomb, level reduction (audibly) zero. Again, 2 50VA toroids, two regulated shunt PSUs.

I'll say it again and take the flack - FORGET op amp data sheets, they lie like dogs regarding available current output, and add a simple 2 stage current boost ciruit, using off the shelf devices. And it will rock!
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Last edited by dvv; 21st December 2012 at 05:20 PM.
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Old 21st December 2012, 05:58 PM   #7884
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I think a good universal buffer with as small a number of components as possible would be worth building . It could be used to drive small and/or efficient speakers / cans even. It could be used with passive pre amps . The challenge would be sound with or without equal or better . It should always be at least as good or better . I reckon 80 % of the time better .

Douglas Self has built a NE5532 power amp . In principle it could be that with better chips . 10 V rms 3 R , job done .
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Old 22nd December 2012, 06:51 AM   #7885
dvv is offline dvv  Serbia
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Well Nige, there are quite a few chip power amps around, I'm told, some delivering quite reasonable power outputs.

When you add an active current gain power stage to a passive preamp, as I see it, it's no longer a passive preamp, just different.

As for op amps as buffers, they can do the job with fyling colours. You have your pick of devices, thank the Lord, some will insist on FET input op amps, I'll insist on AD 829 because of its speed and low noise, etc, there's something for everybody.

What is a "simple buffer"? Highly arguable term. But once again, there's something for everybody. I have generally used three types of buffers. One uses 2 2SK170 FETs and is a variation of the Borbely theme. Another is fully complementary and uses 6 transistors, usually low noise BC 550/560 B types, but while it may appear to be complicated, in fact it's not, it's rather simple and compact. The third uses an AD 829 op amp in unity gain mode. All work well and basically have very little, if any, sound of their own.

I'm quite sure most of us here could tell a story of their own on buffers. The only type of buffer I have not tried is a single tube type, I know far too little about tubes to be able to design anything half decent. And in those days, I didn't know ol' Wave.
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Old 22nd December 2012, 09:26 AM   #7886
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Dvv , my idea is a universal discreet op amp . One which for fun mostly adapt to the use as a small amplifier . It could be for tweeters . As the PCB is accessible some parts could be left out if just a simple buffer required . Range Rover tried to do this with a "one does all " car . Apart from no power take off ( so easy if they wanted to ) it is a near perfect concept , a car that thinks it is a tractor . Like a shopping car we have op amps if not requiring the last word in power output . I get into trouble for talking cars so analogy only this time .

Dvv you said I design by taking pieces out . I would with this . However it must be better than the ready made op amps ( more bias is the easiest route ) . Most of what we see in op amps is because it is always preferable to construct a transistor rather than capacitor or resistor . Given that why do it the simple way when a different world is available ? For example a complex current mirror is no longer a choice , it is the correct choice . Those transistors with >3 junctions , where do they exist outside of op amps ?
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Old 22nd December 2012, 03:30 PM   #7887
dvv is offline dvv  Serbia
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More bias is meaningful only if you have an output stage capable of benefiting from more bias. And soon enough, you realize you might well need heat sinks - that's when simple starts being complex.

Before you know it, your output pair is dissipating more than 1W each, continuous, and they tend to get rather hot at even such relatively small power levels. That's if you want it to be cabale of driving low impedance lines as well, if not, then perhaps two or three small signal BJTs in series/parallel can still do the job.

My experience is that whenever somebody tries to make something adjustable for everybody, you get a jack of all trades and a master of none. It can be good, but it can never rival the best, because the best are highly specialized for one function. This not only means selecting the right trannie for the job, but also optimizing the whole circuit for just those trannies. No small feat.

In my view, you'd be best off designing a discrete op amp in fully complementary topology - you need 10 transistors per channel for this, as I see it. The payback is that you could be able to drive actual real world speakers if you rescale the driver and output BJT pairs. The only limitation is that the pinout of the trannies must be the same.

Even using graden variety transistors, like say BC 550/560 B for input stages, BC 546/556 Bfor the rest and BD 139/140, you would still be able to use PSU lines of +/- 30V.
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Last edited by dvv; 22nd December 2012 at 03:34 PM.
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Old 23rd December 2012, 12:49 PM   #7888
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I sometimes use TO3 because at a pinch they tolerate 2.5W in free air . They look good also . 30 V 5 W = 160 mA standing current . Some of the minimal heat sinks for TO3 are very cheap ( used in TV ) . Might get to 250 mA !!!! Free air like a barbeque is good ( above and below ) .
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Old 24th December 2012, 06:48 AM   #7889
dvv is offline dvv  Serbia
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Well, if they are handy ...

Personally, I much prefer plastic packaging simply because it's so much more convenient. Also, for line level applications and as long as it's not a power amp, I think MJE 15030/15031 is all one could ever wish for. Very well behaved, good beta, all of 8 Amp current capability, 140V, 30 MHz, what else is there?

And because they are relatively small, there's is literally a vast selction of heat sinks, from modest and cheap to even very good, and still cheap.

If you want custom, PADA of Italy, one of Europe's largest heat sink manufacturers, will be glad to comply. Pick a profile, tell them how many holes you want where, they drill them, polish them and anodize at the end. You get a truly professional look. PADA dissipatori per Elettronica, DISSIPATORE di calore
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Old 24th December 2012, 08:03 AM   #7890
dvv is offline dvv  Serbia
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Сретан Божић и Сретну Нову 2013. годину жели Вама и Вашима
Sretan Božić i Sretnu Novu 2013. Godinu želi Vama i Vašima
A Merry Christmas and a Happy New Year to You and Yours from

Dejan V. Veselinović (DVV)
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