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Using the AD844 as an I/V
Using the AD844 as an I/V
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Old 25th May 2018, 02:10 AM   #2221
4rgroup is offline 4rgroup
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Quote:
Originally Posted by georgehifi View Post
I posted this by accident on another thread anyway here it should be.

Funny you should bring up the AD844 Jan.
I found this circuit of Pedja Rodic's (attached) and used it as is (NO FEEDBACK) and it worked a treat, best opamp based I/V so far by a long way, it blew the OPA627 completely away I though it would never be beaten except for a discrete I/V of some sort. (And Ive tried just about every opamp out there)
I used it without feedback as per the diagram and was very very happy. Used with TZ as he has it 1.5K with 1000pf to ground, I think this gives a 1st order rolloff 3db @100khz.
Does the 1.5k resistor have a bearing on what input resistance the dac (PCM1704 in my case) sees?
Because I changed the resistor for 2.2k to get some more gain but this time with a 260pf cap 3db @ 270khz (still all nice and stable with a bit more gain) still very very good.
But I had the feeling it was puchier and more dynamic with the 1.5k resistor even though I had less gain. I feel that the input loading for the PCM1704 is better with the 1.5k than the 2.2k resistor or am I imagining things?.

Cheers and thanks for your input George
Hi George,

I want to apply this solution for my TDA1541A DAC.
Could I use a VR, say 3K ohm, instead of 1.5k resistor at TZ pin as a volume control for the DAC? I want to connect the DAC to power amp (sunfire 300) without a preamp. Is it possible? Can I have your advises this case?

Thanks and Regards
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Old 25th May 2018, 03:19 AM   #2222
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by 4rgroup View Post
Hi George,

I want to apply this solution for my TDA1541A DAC.
Could I use a VR, say 3K ohm, instead of 1.5k resistor at TZ pin as a volume control for the DAC? I want to connect the DAC to power amp (sunfire 300) without a preamp. Is it possible? Can I have your advises this case?

Thanks and Regards
Can't see why not, just use a 220pfcap across it for a -3db at 241khz. Watch you don't overload the input of the sub with too much gain.

Cheers George
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Last edited by georgehifi; 25th May 2018 at 03:22 AM.
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Old 25th May 2018, 06:38 AM   #2223
canvas is offline canvas  Taiwan
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Glad to see this thread being alive again ^_^

This IV resistor is critical. IMHO, using a VR may not be an optimal solution. Besides that, you need to look into Sunfire's input biased current. This setup could be too weak to drive an amplifier when VR is at low resistance.
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Old 25th May 2018, 07:18 AM   #2224
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Originally Posted by canvas View Post
Glad to see this thread being alive again ^_^

This IV resistor is critical. IMHO, using a VR may not be an optimal solution. Besides that, you need to look into Sunfire's input biased current. This setup could be too weak to drive an amplifier when VR is at low resistance.
The Sunfire has the RCA Input impedance of 24K ohm (from the manual)
Now, I use 22 ohm as I/V resistor then a SRPP 6Dj8 tube buffer which has gain of 25, output resistor of 500 ohm and Pmax of 0.9W. I want to insert AD844 as a IV stage (replacing for I/V resistor) and BTW a volume control here. The reason for doing that is to remove the next preamp from the system.

I want to remove the SRPP tube buffer and use 844 as a buffer too. However I do not see where to insert the VR here this way

I am a newbie, please advised me.

Thanks a lot.
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Old 29th May 2018, 03:00 PM   #2225
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Hi 4rgroup,

I've never thought about that. I think the performance will greatly affected by the quality of VR and the wiring. You can try to use the VR on pin5 and take pin6 as the output to the amplifier. If you don't take pin6 as the output, an external buffer is recommended. There are many options here. You may read other's feedback on this thread.

Poting
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Old 8th June 2018, 09:00 AM   #2226
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Originally Posted by georgehifi View Post
Can't see why not, just use a 220pfcap across it for a -3db at 241khz. Watch you don't overload the input of the sub with too much gain.

Cheers George
Dear George,
I have only one AD844 so far. So I've turned to Discrete Diamond Non-Feedback I/V Stage circuit (DDNF) as a I/V stage in my DAC.
It's schematics can be found here: http://www.diyaudio.com/forums/blogs...f-iv-stage.pdf
I aslo use a SRPP SRPP 6Dj8 tube as the gain stage. The TDA1541A’s output phase is inverted (am I right?) and the SRPP gain stage inverts the phase too. If the DDNF's phase is not inverted, the DAC as a whole has correct polarity.
My question is that the DDNF's output phase is inverted or non-inverted. Could you kindly advise me.

Thanks and Regards.

Last edited by 4rgroup; 8th June 2018 at 09:19 AM.
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Old 15th June 2018, 07:23 AM   #2227
triode_al is offline triode_al  Netherlands
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Currently I have had a BUF03 as buffer, but I want to replace it. I also want to make a balanced driver.
Many have used the OPA627.
I see that the OPA1612 is newer and part of the SoundPlus family of opamps - and more widely available. Both are unity gain stable.
What are your experiences.
Should I be aware of fakes (some Ali prices are lower than others)?
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Last edited by triode_al; 15th June 2018 at 07:36 AM.
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Old 15th June 2018, 07:33 AM   #2228
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Quote:
Originally Posted by canvas View Post
Glad to see this thread being alive again ^_^

This IV resistor is critical. IMHO, using a VR may not be an optimal solution. Besides that, you need to look into Sunfire's input biased current. This setup could be too weak to drive an amplifier when VR is at low resistance.
What about using a variable resistor at 1k5 (the intended I/V value) and then just using the tap as volume control?
The wiper must be loaded high (for optimal response) otherwise there will be sagging at higher volume settings.
That apart from the quality of course, but some studio VR could have a high non-inductive quality I expect.
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Old 15th June 2018, 08:11 AM   #2229
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Quote:
Originally Posted by triode_al View Post
What about using a variable resistor at 1k5 (the intended I/V value) and then just using the tap as volume control?
The wiper must be loaded high (for optimal response) otherwise there will be sagging at higher volume settings.
That apart from the quality of course, but some studio VR could have a high non-inductive quality I expect.
yeah I mentioned someone should try this 6mts ago.

Cheers George
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Old 20th June 2018, 09:04 PM   #2230
Hierfi is offline Hierfi  Canada
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Originally Posted by Joe Rasmussen View Post
In the 844 you have two parts, you have the OTA front-end and in the 844 it runs at a fixed current and I estimate it might be only 2mA. By stacking the 844, the OTA has a lot more current, not just the other part, the buffer. In the 861 you can set it by a single resistor and easily run 3-4 times the current through the OTA part.

Mind you, the 861 is really meant for low -/+5V operation and the 844 can handle -/+15V and a bit more OK. So there are more variables there.

Cheers, Joe
I am interested in the current passing through the transistors on the inputs of the AD844 to determine its limits in class A.

Is it reasonable to guess that there are three paths of equal current between the power supplies... excluding the output stage? Two paths on either side of the input quad and a current mirrored from the inverting terminal? If the output stage current is included in the total current of 6.5mA this suggests perhaps 1.5mA in each of the three legs. This is dependent upon the output stage buffer current consuming 2mA. Is the actual current information available anywhere?

Cheers,Gerrit
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