increase the bias current of an opamp final stage - Page 2

peranders · 12th September 2003, 02:44 PM

If you want good quality gain of 1000 an opamp in this category will be good and without a buffer. You need a buffer if you need more output current than 10-20 mA. Most opamps can handle 10 volts into 1 kohms loads.

Low noise opamps
LT1037
LT1028
LT1128
LT1115
AD797
OPA627
NE5534 (a litlle bit old but worth the money, I can sell those for 3 SEK a piece)

Not so low noise
OPA134
AD8610
plus many many more

Did I mention that I have LT1028 for sale?

1 USD = 7 SEK (including exchange)
1 EUR = 8 SEK (including exchange)

Dan Banquer · 12th September 2003, 02:47 PM

You might try something like this.
http://www.digido.com/User/Assets/Ac...478-buffer.pdf
I think this will easily satisfy line drive requirements and drive a headphone amp.
One of the advantages of a topology like this is that the the op amp "see's" a predominantly resistive load. The output buffer deals with the real load. This allows you to use "lightweight" drive op amps and turn them into a killer line driver.

carlmart · 12th September 2003, 03:45 PM

Quote:

Originally posted by herve
Well,an option on the pcb to add a 68 ohm resistor it's maybe a good idea.
But for Q1,Q2 do i need a pair of transistors in the same package like with an LM394?

The purpose of this is not for making an headphone amplifier.
The amplification factor is around 1000.
Maximum output around 1 volt.
Load: 10 to 100 kilo resistor.

The "idea" is to make a final stage with transistors which work "far away" from the cutoff current..

Most discrete line preamps are usually designed to work in class-A, even if that is usually stated when you have to handle high currents or drive difficult loads.

Using a mated pair for the input transistors is certainly a good idea.

That ESP discrete preamp has an option to null offset, as you can see from the article, and you should use it. Perhaps an output capacitor, if your power amp does not have one, is probably a good idea.

Rod also warns that input impedance is not so good, but as long as you do not feed high impedance into it you should be fine. Most sources nowadays are low impedance anyway.

Rod also thinks this preamp is in a lower league than an NE5532, for instance, but whether if it sounds better or not is something to be found out. He is an "objectivist", if such a thing exists, so he mostly refers to data comparison for that statement.

Carlos

herve · 12th September 2003, 04:29 PM

Well,it'is not a matter of class(A,AB).
The idee is:if i have a lot of current bias,the swing current will be a very small change in my circuit.
It is or not a good idea?

I have a question to Per-Anders.
I have seen (if i make no mistake),that you use in your phono amplifier a buffer and class a circuit:

http://home.swipnet.se/~w-50719/hifi...ema_qsxm3.html

and this is why i wonder at that kind of solution.

Herve.

sonnya · 12th September 2003, 04:35 PM

Quote:

Originally posted by PMA
ACD,

that's just a basic, nothing more (DIYopamp). 34 years ago there was a better example of Sinclair Z-30 or Z-50.

I did not know that Audio precision did exist 34 years ago

JOE DIRT® · 12th September 2003, 06:02 PM

LOL ACD...there is simply no respect for old timers around here

Cheers!!DIRT®

john curl · 12th September 2003, 09:04 PM

Herve, your intuition is correct. However; the alternate circuits presented are not optimum. The all discrete circuit is too crude. The added current source in the second example is too complex for just an IC. However, the second alt will work OK, except that discrete is better.

herve · 13th September 2003, 09:27 AM

I am thinking at something like this:

PMA · 13th September 2003, 11:04 AM

herve, should look like this to have a chance to work:

herve · 15th September 2003, 12:29 PM

Well,i have try to make a kind of classification...
(I don' talk about feedback to stay simple).

I can use a good quality opamp like Per Anders suggest me.

The "output voltage" is very low and it seems not necessary to bias the opamp into Class A.

well if i use a buffer ,i can add a switch to bypass it and see what is the best way.

i can use in first time a very simple buffer like jam

( Walt Jung IC opamp/buffer combo vs. discrete?)

and in second time a buf 634 like ppl

DIY preamp design - gain topology, IC vs. discrete, OPA627 vs. Zen... ?

well i can do or draw like sonnya a complete discrete solution if i have more time (and/or money) and more knowledge.

Jfet/bipolar PCB and Schematic (finaly)

It seems to me that ,Rod Elliott or Nelson Pass discrete opamp have been not draw for an application like me.

Tell me if i have made a mistake.
Thank you.

12th September 2003, 02:44 PM	#11
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	If you want good quality gain of 1000 an opamp in this category will be good and without a buffer. You need a buffer if you need more output current than 10-20 mA. Most opamps can handle 10 volts into 1 kohms loads. Low noise opamps LT1037 LT1028 LT1128 LT1115 AD797 OPA627 NE5534 (a litlle bit old but worth the money, I can sell those for 3 SEK a piece) Not so low noise OPA134 AD8610 plus many many more Did I mention that I have LT1028 for sale? 1 USD = 7 SEK (including exchange) 1 EUR = 8 SEK (including exchange) __________________ /Per-Anders (my first name) or P-A as my friends call me Tube Buffered Gainclone in work \|Thread

12th September 2003, 02:47 PM	#12
Dan Banquer diyAudio Member Join Date: Aug 2003 Location: Swampscott MA	Buffers You might try something like this. http://www.digido.com/User/Assets/Ac...478-buffer.pdf I think this will easily satisfy line drive requirements and drive a headphone amp. One of the advantages of a topology like this is that the the op amp "see's" a predominantly resistive load. The output buffer deals with the real load. This allows you to use "lightweight" drive op amps and turn them into a killer line driver. __________________ I was an audiophile until I found out what they were doing in the recording studio.

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12th September 2003, 04:29 PM	#14
herve diyAudio Member Join Date: Jan 2003 Location: france	Well,it'is not a matter of class(A,AB). The idee is:if i have a lot of current bias,the swing current will be a very small change in my circuit. It is or not a good idea? I have a question to Per-Anders. I have seen (if i make no mistake),that you use in your phono amplifier a buffer and class a circuit: http://home.swipnet.se/~w-50719/hifi...ema_qsxm3.html and this is why i wonder at that kind of solution. Herve.

12th September 2003, 06:02 PM	#16
JOE DIRT® diyAudio Member Join Date: Apr 2003 Location: Brantford, ON	LOL ACD...there is simply no respect for old timers around here Cheers!!DIRT®

12th September 2003, 09:04 PM	#17
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	Herve, your intuition is correct. However; the alternate circuits presented are not optimum. The all discrete circuit is too crude. The added current source in the second example is too complex for just an IC. However, the second alt will work OK, except that discrete is better.

15th September 2003, 12:29 PM	#20
herve diyAudio Member Join Date: Jan 2003 Location: france	Well,i have try to make a kind of classification... (I don' talk about feedback to stay simple). I can use a good quality opamp like Per Anders suggest me. The "output voltage" is very low and it seems not necessary to bias the opamp into Class A. well if i use a buffer ,i can add a switch to bypass it and see what is the best way. i can use in first time a very simple buffer like jam ( Walt Jung IC opamp/buffer combo vs. discrete?) and in second time a buf 634 like ppl DIY preamp design - gain topology, IC vs. discrete, OPA627 vs. Zen... ? well i can do or draw like sonnya a complete discrete solution if i have more time (and/or money) and more knowledge. Jfet/bipolar PCB and Schematic (finaly) It seems to me that ,Rod Elliott or Nelson Pass discrete opamp have been not draw for an application like me. Tell me if i have made a mistake. Thank you.