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Old 12th February 2019, 11:38 PM   #11
johnego is offline johnego  Indonesia
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If you want to use the amp both for speaker and hp, you can add dummy load just like traditional amplifier, to keep the load impedance around 8 ohm. But if you want to use it only for hp then you can modify the amp.

C3 is to make sure there is constant voltage across base resistor. This in turn create constant base current. This base current, with hfe, turns top transistor into constant current source to make distortion lower. But you cant just reduce the cap without affecting bass roll off. There is a minimum (RC) value.

To solve your current issue, i think you can reduce the overall gain, which you don't need much if only for hp. Or, may be you can move the bootstrap point closer to the rail. Your base resistors are 220 and 400, you can change this probably into something like 100 and 520. The total resistance determine the base current.
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Old 13th February 2019, 02:40 AM   #12
nauta is offline nauta  Canada
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Hi
JLH used this circuit for many low-wattage things, like driving tube grids and as a headphone amp. Why not build a low-watt hp amp and optimise the compensation for the desired load?
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Old 14th February 2019, 12:08 AM   #13
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by mjona View Post
I still have the rather complex pre-amplifier published in Wireless World in 1982 which includes one of these headphone amplifier examples - this based around an IC op.amp and a couple of medium power transistors.....
Hi Michael, can you tell us the particular Wireless World issue you refer to? We have an excellent archive at THE HISTORY OF RADIO documented in thousands of PDF books and magazines but sifting through the issues isn't cheap or easy.
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Old 14th February 2019, 12:34 AM   #14
johnego is offline johnego  Indonesia
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I think mjona gave the link to Paul Kemble website with said preamp (Tone Control) and head amp. November issue. And a matching 80W mosfet amplifier in August issue. Nothing special about op amp and the circuit.
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Old 14th February 2019, 03:21 AM   #15
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by Ian Finch View Post
Hi Michael, can you tell us the particular Wireless World issue you refer to? We have an excellent archive at THE HISTORY OF RADIO documented in thousands of PDF books and magazines but sifting through the issues isn't cheap or easy.
The circuit diagram is on page 63 of the November 1982 issue. The layout of the pcb is on page 80 of the February 1983 issue.
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Old 14th February 2019, 05:59 PM   #16
Ian Finch is offline Ian Finch  Australia
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Many thanks for the directions. For quite some years, I've also enjoyed the major improvement of a low impedance drive for headphones. I'm looking forward to testing a lash-up of this one as I don't think I've ever tried anything quite as simple and potentially quite small. If it still works well enough, this always make things more attractive
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Old 15th February 2019, 06:32 AM   #17
steveu is offline steveu  United States
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On the negative peak, Q4 is turning Q2 back on because the 600 Ohm load does not draw all the Q4 current. You can adjust R7 so that Q4 comes on when Q2 Vb is at half V1, but this does not work when you have a reactive load or not exactly the predicted load value. A successful class AB output is designed so that the crossover can happen at any voltage. This is 2019 so I suggest you buy some xx2955 PNP power transistors and make this dog of a circuit into one that works well.
BTW, I have a circuit that uses only NPN transistors that simulates well, but it is only useful for chip amps where pnp transistors are a problem. Given the availability of good cheap PNPs, it's pointless.
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Old 16th February 2019, 05:33 AM   #18
mjona is offline mjona  New Zealand
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I have attached the circuit using a dual IC and a couple of medium power transistors that I mentioned earlier. It should work in Class A under most conditions - lower impedance headphones generally require a lower voltage swing which will allow the latter mode of operation.

Higher impedance headphones may require a power amplifier to drive them. So if there is a problem with that replace the 600 Ohm head phones with a low impedance set.

It seems from the discussion of a simulation the point of building the hardware has not yet arrived and I thought there might be some interest in a simple low cost way to resolve the issue - offsetting the cost of replacing the headphones.
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Old 16th February 2019, 09:14 AM   #19
kgrlee is offline kgrlee
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Quote:
Originally Posted by steveu View Post
A successful class AB output is designed so that the crossover can happen at any voltage. This is 2019 so I suggest you buy some xx2955 PNP power transistors and make this dog of a circuit into one that works well.
Err.rrh! JLH 1969 isn't Class AB but full Class A

I don't think you understand why it is still so popular.

Its THD profile, 0.1% almost pure 2nd, near clipping and reducing rapidly with level is one of the few 'inaccurate' amps that may actually be preferred .. even by true golden pinnae who recognise that it isn't fully 'accurate' in DBLTs.

It's quite difficult to replicate this THD profile .. especially with speaker loads. You should check this out in SPICE world and see if you can make a Class AB amp with your extra xx2955 to 'sound' as good.

If you want accurate, build an O2 amp.

Some of JLH's later headphone amps .. like the one mjona just posted ... may be 'Class A' but are quite horrible for a number of reasons and aren't accurate either.

Last edited by kgrlee; 16th February 2019 at 09:17 AM.
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Old 16th February 2019, 05:48 PM   #20
mjona is offline mjona  New Zealand
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Quote:
Originally Posted by kgrlee View Post
Some of JLH's later headphone amps .. like the one mjona just posted ... may be 'Class A' but are quite horrible for a number of reasons and aren't accurate either.
Without stating these reasons I see your comments as a slur - you are not saying you have you actually built any of these circuits or listened to them.

I gave the specifications for the circuit in question in post 9 you have given no technical argument to counter Linsley-Hood's results.

What are you on about?
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