Hi, reasonably new to solid state & op amps; just wondering if anyone could suggest a good op-amp option(s) for a design I'm working on.
Input: Needs to handle a tube preamp stage, can either be high impedance/large signal (up to 80VAC) or low impedance/smaller signal (2-3VAC)
Power Supply: +/-18.5V @ 150mA
Output: Needs to drive a 2.8kOhm input impedance up to 10VAC
Please let me know if you have any thoughts, schematics or recommended gain stage would be extremely appreciated. Thanks!
Input: Needs to handle a tube preamp stage, can either be high impedance/large signal (up to 80VAC) or low impedance/smaller signal (2-3VAC)
Power Supply: +/-18.5V @ 150mA
Output: Needs to drive a 2.8kOhm input impedance up to 10VAC
Please let me know if you have any thoughts, schematics or recommended gain stage would be extremely appreciated. Thanks!
Hummm… like almost ANY op-amp?
You know … stating an input of 80 VAC in 'high impedance' acceptance and then saying ±18 V power is kind of a head-knocker.
Is the purpose of this OpAmp to be 'just a buffer', which is to say, no voltage gain, but impedance lowering (for the 2.8 kΩ input you imagine down-line of it)?
And what is a 2.8 kΩ input impedance device?
Sounds really, really low to me.
Oh well.
Things.
To.
Think.
About.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
You know … stating an input of 80 VAC in 'high impedance' acceptance and then saying ±18 V power is kind of a head-knocker.
Is the purpose of this OpAmp to be 'just a buffer', which is to say, no voltage gain, but impedance lowering (for the 2.8 kΩ input you imagine down-line of it)?
And what is a 2.8 kΩ input impedance device?
Sounds really, really low to me.
Oh well.
Things.
To.
Think.
About.
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
80 vac * 1.4 = 112 v dc. there is no op amp made that will take 112 vdc on input. Amps subject to having a 35 vac signal plugged into the input, have it protected by diodes to the power supply, with a 1/10 w 1000 ohm resistor used as a fuse to blow if some drunk or dark blinded roady does that.
Op amps have high impedance input and low impedance output.
If your 80 vac signal is a given, then you need a transformer to reduce the voltage level down to something that will fit between the power supplies. The highest power supply op amp I'm aware of is the LM675 for +-30 v. Or single supply 60 v.
There were vacuum tube op amps in the 60's but philbrick got bought up. Had horrible DC stability problems too, solved by complicated chopper circuits that could inject noise into audio circuits.
Most op amps can drive 2.8kohms at 10 vac. I like the 33078 my self, quiet at high gains. Unity gain, more like a 4558 or something.
With that low load, a transformer could do both jobs on the 80 vac signal. The 3 vac signal, from a different jack, could be a 4560 or 4580 even. Just watch out for gains below 5, most op amps don't like it.
Op amps have high impedance input and low impedance output.
If your 80 vac signal is a given, then you need a transformer to reduce the voltage level down to something that will fit between the power supplies. The highest power supply op amp I'm aware of is the LM675 for +-30 v. Or single supply 60 v.
There were vacuum tube op amps in the 60's but philbrick got bought up. Had horrible DC stability problems too, solved by complicated chopper circuits that could inject noise into audio circuits.
Most op amps can drive 2.8kohms at 10 vac. I like the 33078 my self, quiet at high gains. Unity gain, more like a 4558 or something.
With that low load, a transformer could do both jobs on the 80 vac signal. The 3 vac signal, from a different jack, could be a 4560 or 4580 even. Just watch out for gains below 5, most op amps don't like it.
Yup, I'm basically just trying to connect the output of a 12AX7 with Ice Power's 100AS1 Power Module...I initially hooked it up like a tube amplifier's power section but couldn't get full power output...after doing some research and help from this site it sounds like the large impedance mismatch is the issue. The 100AS1 works fine with my signal generator.
Here's the 100AS1 datasheet :
http://app.efo.ru/storage/ice/seria_as/ICEpower100AS1-2-Datasheet_1_4-1.pdf
Input Impedance is listed on page 10, maybe I misinterpreted.
So yeah, if using the tube's cathode for a lower impedance signal is better then that's cool - I was just putting all options out there to not limit solutions. The 100AS1 supplies +/-18.5 volts from the board so I'm hoping to find an op-amp that works in that range, just not sure where to start.
Here's the 100AS1 datasheet :
http://app.efo.ru/storage/ice/seria_as/ICEpower100AS1-2-Datasheet_1_4-1.pdf
Input Impedance is listed on page 10, maybe I misinterpreted.
So yeah, if using the tube's cathode for a lower impedance signal is better then that's cool - I was just putting all options out there to not limit solutions. The 100AS1 supplies +/-18.5 volts from the board so I'm hoping to find an op-amp that works in that range, just not sure where to start.
The 12AX7 output of my PAS2 preamp puts out a 2 vac signal. 250 v plate, 2vac signal. You need to clip or otherwise limit the input to the 12AX7 to keep it from putting out too much signal. Look at dynaco PAS3 schematic all over the internet.
Had dynaco spent $6 per channel on transformers, the 12AX7 could have driven more RCA coax cable than 6'. But the market said $120 kit, 6' cable to power amp. they sold hundreds of thousands of them.
Had dynaco spent $6 per channel on transformers, the 12AX7 could have driven more RCA coax cable than 6'. But the market said $120 kit, 6' cable to power amp. they sold hundreds of thousands of them.
Not all, at all.
Inverting Op amps both have lowish input impedance and can handle *any* input signal voltage, just size input resistor accordingly.
And as stated, it can also have high input impedance (unless you consider 1M "low")
An inverting Op Amp with 1M input resistor and 33k feedback one, will satisfy 3 conditions:
* High input impedance (1M)
* will easily handle 80V RMS
* under such input, will put out 4V RMS, more than enough to drive the 2.8V sensitive input.
As a bonus, if you control gain by using a 50k Log pot as NFB, you both optimize gain and signal handling, how´s that?
Not necessary at all, please read above for my suggestion.
The oldest IC op amp I can find a datasheet for, lm101a has maximum 100 na input current over temperature. So max signal 15v/100na=150 megohm input impedance. I call that high impedance, I'm sorry. Base input of common emitter transistors on power amp inputs are more like 100 k to 150 kohms I'd call that medium impedance.
The op amp circuit can have low impedance, due to the connection of the other input to analog ground with a low resistance, but the op amp IC doesn't.
Nice circuit as described, but seems a bit dangerous to the IC.
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It might be wise to pick a FET input opamp in that arrangement as that minimizes current noise.
And of course an inverting stage inverts phase. You could use a 1M:33k divider into a non-inverting follower for a non inverting alternative.
+/-18.5V is just beyond the abs-max voltage of many opamps, which are designed for +/-15V. A pair of 7815 and 7915 regulators could fix that if relevant.
LM4562 is a pretty nice chip, though it has a pretty low input impedance (at least for an op-amp).
OPA604 is rated for up to +/- 24V. Distortion and noise performance isn't groundbreaking, but the relatively high voltage rating can be useful. Some people like to describe them as "musical" or what have you, but I decline to comment on descriptions that use useless adjectives like that.
Even a TL072 can have very good performance if used within it's limits. Don't fall into the trap of thinking that high-end, expensive chips are a substitute for good circuit design.
OPA604 is rated for up to +/- 24V. Distortion and noise performance isn't groundbreaking, but the relatively high voltage rating can be useful. Some people like to describe them as "musical" or what have you, but I decline to comment on descriptions that use useless adjectives like that.
Even a TL072 can have very good performance if used within it's limits. Don't fall into the trap of thinking that high-end, expensive chips are a substitute for good circuit design.
LME49860 if I recall is good to +/- 22V, but I dont think they're sold any longer.
Theres also something like OPA548
Theres also something like OPA548
Oh they are still sold by authorized distributors and are not marked EOL, I actually just ordered a few. I don't know if they are still sold in the DIP-8 package or only SOIC-8 though.