That Corp 1606 after Pot

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RSRecords

Well-known member
Joined
Jun 8, 2009
Messages
319
Would I need a unity buffer amp between a 10k pot (fader) and a that Corp balanced line driver?
 
It is pretty common to put 10dB of gain in a post fader stage. That gain stage could be your low z buffer.
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From the data sheet  http://thatcorp.com/datashts/THAT_1606-1646_Datasheet.pdf 

THAT says said:
2. Both devices (1606/1646) must be driven from a low- impedance source, preferably directly from opamp outputs, to maintain the specified performance.

I find reading manufacturer data sheets useful.

JR
 
So i was thinking something simple like this. Or would it be better as an inverting amp?

haven't worked out values yet but I think I'll only need 3-4db of gain since the 1646 has an additional 6db.



also, I don't think the  series resistors from  the wiper are doing anything  positive so disregard.

 

Attachments

  • Stereo unbal Faderschem.pdf
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I tried it recently with a THAT 1646, placing a pot right before the input.  The result was ugly to say the least.
 
CurtZHP said:
I tried it recently with a THAT 1646, placing a pot right before the input.  The result was ugly to say the least.


makes sense. I little opamp buffer is the way to go i guess..

looking at resistor values:
G=1+r2/r1
4dB=1.58

so thinking R2=16.8k (or 16.9k seems to be a standard)
r1=10k

I understand there are a few things to consider regarding that ratio. Any benefit to going lower say 5k, and 8.4k?

what would I be looking at for R3? i guess I'm unsure as to what it would be doing.
 
RSRecords said:
I understand there are a few things to consider regarding that ratio. Any benefit to going lower say 5k, and 8.4k?
It depends on the opamp. If you stick to OPA2134, it just doesn't matter. An opamp with significant Input Noise Current may want lower values. FET opamps have negligible INC.

what would I be looking at for R3? i guess I'm unsure as to what it would be doing.
I'd put anything between 1k and 4.7k. That's enough to ensure it imposes no restriction on HF response (is 200kHz enough?), whilst protecting the non-inverting input from whatever HF pollution.
Again, my answer would be different if the opamp was a 5532.
 
ok so I think this should do it. Added filtering after the input resistors (100pf). Went with 3.3K for R3. should be ~48kHz filter I think.
Thinking an100pF cap in the feedback path. Can always omit if not needed.
 

Attachments

  • faderschem.pdf
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abbey road d enfer said:
No. The days when inverting was better than non-inverting are long gone.

When were those days? And why?, I mean I know that non-inverting can cause common mode distortion if driven with significant source impedance (this is still a problem today), but cant think of why inverting should be considered better at some point in time?
 
user 37518 said:
When were those days? And why?, I mean I know that non-inverting can cause common mode distortion if driven with significant source impedance (this is still a problem today), but cant think of why inverting should be considered better at some point in time?
You answered your own question. Opamp designers have dealt with this issue over the last 50 years.
 
So, finally got around to building this with the above circuit. How can I increase the Kill of the fader? Getting a little bit of signal still when all the way down. I haven't measured yet to give an actual number. The fader is actually a 10k Pot so it doesn't have a switch or a fancy ground scheme like a P&G.
 
abbey road d enfer said:
You answered your own question. Opamp designers have dealt with this issue over the last 50 years.
+1 Inverting is easier for multiple reasons.

Operating non-inverting with high voltage gain is easier on the op amps because the input stage is operating over a smaller voltage range.

Simple unity gain followers are the worst case, but even they can be reasonable with modern op amps as long as you stay away from the rails, and some modern ones even tolerate that.

JR (aka  TMI).
 
RSRecords said:
So, finally got around to building this with the above circuit. How can I increase the Kill of the fader? Getting a little bit of signal still when all the way down. I haven't measured yet to give an actual number. The fader is actually a 10k Pot so it doesn't have a switch or a fancy ground scheme like a P&G.
What happens if you short the wiper to ground (with the pot full CCW)? If there is still spill, I would suspect the grounding arrangement. There could also be capacitive leakage. Measuring would help pinpoint the issue; "ground" spill is linear, capacitive is HF only.
 
So, it seems it was grounding. I increased the wire gauge and moved it closer to the driving amps ground. Works like a champ now.

Thanks for the help
 
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