Hi,
Here's my problem, in this diagram...
I have an output that is slightly too hot for the device that uses this circuit's output. The signal is about 2V peak to peak.
The first opamp is just a unity gain voltage follower. Therefore the input signal must also be about 2v peak to peak. The input is a piezo pickup, so the opamp must provide a Hi input Z.
What would be the best option to bring get that output signal down a little without affecting the following opamp path? (that said, if needs must & I have no other option, I can accept the attenuated signal impacting the following opamp circuit too!)
The device that uses the circuit's output is expecting the output impedance of this circuit to be 100 ohms.
I had thought of say a '10k preset' between the output & ground with the preset wiper being the new output...that will obviously reduce the signal, but I'm not sure how that affects the output impedance (or if it'll mess with the later opamp's config.
Here's my problem, in this diagram...
An externally hosted image should be here but it was not working when we last tested it.
I have an output that is slightly too hot for the device that uses this circuit's output. The signal is about 2V peak to peak.
The first opamp is just a unity gain voltage follower. Therefore the input signal must also be about 2v peak to peak. The input is a piezo pickup, so the opamp must provide a Hi input Z.
What would be the best option to bring get that output signal down a little without affecting the following opamp path? (that said, if needs must & I have no other option, I can accept the attenuated signal impacting the following opamp circuit too!)
The device that uses the circuit's output is expecting the output impedance of this circuit to be 100 ohms.
I had thought of say a '10k preset' between the output & ground with the preset wiper being the new output...that will obviously reduce the signal, but I'm not sure how that affects the output impedance (or if it'll mess with the later opamp's config.
You can get 10 dB of attenuation if you change the value of R2 to 1000 ohms and then put a 100 ohm resistor from the output of R2 to ground.
Hi Frank,
Many thanks,
Forgive my ignorance (never did get to grips with dB)....
What kind of drop in voltage terms can I expect to see?
...eg if the signal was 2V peak to peak before your suggested attenuation mod, what (approx) will it be afterwards?
Also, I'm guessing(!) that the 100 ohm resistor after R2 to gorund maintains the 100 Ohm impedance that the unit this signal feeds is expecting?
Many thanks,
Forgive my ignorance (never did get to grips with dB)....
What kind of drop in voltage terms can I expect to see?
...eg if the signal was 2V peak to peak before your suggested attenuation mod, what (approx) will it be afterwards?
Also, I'm guessing(!) that the 100 ohm resistor after R2 to gorund maintains the 100 Ohm impedance that the unit this signal feeds is expecting?
With 2.0 volts going into the pad, the output wuill be .63 volts.
If you need less attenuation, you can change the value of R2.
The output impedance is controlled by the combination of the 1000 ohm resistor and the 100 ohm resistor.
With this combination, the actual output impedance is 90 ohms.
If you need less attenuation, you can change the value of R2.
The output impedance is controlled by the combination of the 1000 ohm resistor and the 100 ohm resistor.
With this combination, the actual output impedance is 90 ohms.
Thanks Frank - very helpful.
That amount of attenuation is a lot more than I was wanting - I'd be shooting for about 1.5V, therefore what is the formula can I use to establish the attenuation (so I can have less!)
Also, what formula are you using to establish the output impedance?!!!
Very basic stuff I'm sure - but nevertheless, we've all got to learn!
That amount of attenuation is a lot more than I was wanting - I'd be shooting for about 1.5V, therefore what is the formula can I use to establish the attenuation (so I can have less!)
Also, what formula are you using to establish the output impedance?!!!
Very basic stuff I'm sure - but nevertheless, we've all got to learn!
My math was wrong. Sorry, I't early in the morning here.
With R2 at 1000 ohms and an additional 100 ohms to ground the actual output voltage will be about .2 volts. I don't know what I was thinking (or perhaps not thinking).
If you keep the resistance of R2 at 100 ohms and place a 330 ohm resistor between the output of R2 and ground, you will get about 1.5 volts out and keep the output impedance low.
This is a good link to help you calculate just about anything.
http://www.calculatoredge.com/
With R2 at 1000 ohms and an additional 100 ohms to ground the actual output voltage will be about .2 volts. I don't know what I was thinking (or perhaps not thinking).
If you keep the resistance of R2 at 100 ohms and place a 330 ohm resistor between the output of R2 and ground, you will get about 1.5 volts out and keep the output impedance low.
This is a good link to help you calculate just about anything.
http://www.calculatoredge.com/
Level of attenuation is simple potential divider action - you can find this easily so I won't repeat it here.
Output impedance is just R2 in parallel with R3, 100 and 330 in the last example giving 76.7 ohms.
Output impedance is just R2 in parallel with R3, 100 and 330 in the last example giving 76.7 ohms.
Thanks richie00boy.
76.2 ohms is correct. If you actually need an output impedance of 100 ohms, you can simply add 23.8 ohms in series with the output.
I suspect that the output impedance isn't too critical but I may be wrong.
76.2 ohms is correct. If you actually need an output impedance of 100 ohms, you can simply add 23.8 ohms in series with the output.
I suspect that the output impedance isn't too critical but I may be wrong.
If the series resistor is R2 (as in your diagram) and the shunt resistor from o/p to ground is R3, then
attenuation factor A = R3 / (R2 + R3)
output impedance Z = (R2 x R3)/(R2 + R3)
To help you out I have rearranged these so you can get the values of R2 and R3 you want straight from the attenuation and impedance you want:
R2 = Z / A
R3 = Z / (1 - A)
Eg: if you want an attenuation of 0.8 and an impedance of 100 ohms, you need to make R2 = 125 ohms and R3 = 500 ohms.
attenuation factor A = R3 / (R2 + R3)
output impedance Z = (R2 x R3)/(R2 + R3)
To help you out I have rearranged these so you can get the values of R2 and R3 you want straight from the attenuation and impedance you want:
R2 = Z / A
R3 = Z / (1 - A)
Eg: if you want an attenuation of 0.8 and an impedance of 100 ohms, you need to make R2 = 125 ohms and R3 = 500 ohms.
traderbam said:
Traderbam - that was exactly the info I wanted - fantastic!
Many thanks too, to all those who took the time & trouble to reply here.
how about C to gnd on the input? - forms a AC divider with the pickup so a small hi quality film or NP0/C0G would work
could even use a trim Cap to adjust level ("possible microphonic" effects? )
doesn't load the op amp
could be built into the pickup or cable connector
could even use a trim Cap to adjust level ("possible microphonic" effects? )
doesn't load the op amp
could be built into the pickup or cable connector
traderbam said:
Excellent point!
You had better check what the minimum load is, Hank.
It's a TL074 - I just downloaded the datasheet & my head spontaneously combusted at the sheer quantity of info (I couldn't find a reference to minimum output load)
the TL07x series is notorious for not having a beefy output stage.
i wouldn't recommend a 600 ohm load for that part.
unfortunately, i haven't really kept up with quad op amps, so i can't recommend a suitable replacement.
sorry,
mlloyd1
i wouldn't recommend a 600 ohm load for that part.
unfortunately, i haven't really kept up with quad op amps, so i can't recommend a suitable replacement.
sorry,
mlloyd1
Humour me here (genuinely just trying to get this right in my head)...
I can readily change R2 - so what would be the best option wrt resistor combination/solutions on the first opamp output that would satisy the attenuation I seek, an output impedance (of 100 ohms for the follow on device) & an output load that the opamp will be ok with?!
Incidentally, what would be the minimum load recommendation for the TL07x series? If I have this figure - I can work from there!
I can readily change R2 - so what would be the best option wrt resistor combination/solutions on the first opamp output that would satisy the attenuation I seek, an output impedance (of 100 ohms for the follow on device) & an output load that the opamp will be ok with?!
Incidentally, what would be the minimum load recommendation for the TL07x series? If I have this figure - I can work from there!
If you can accomodate one more op-amp then you can make the impedance problem go away, just make the potential divider a more usable impedance then buffer that.
alas I can't (well not without a *lot* of rehashing, extra circuit board etc!).
If I'm reading this right - you're all saying that the load shouldn't be as low as 600 ohms - therefore a higher load is better?
My question is - at what point higher up the scale should I choose as a bare minumum (load wise) for a TL07x?
PS. This actual audio signal won't be heard - the output I show actually feeds a device that anaylyses the frequency/level content. The device I mention comes with a software editor interface which has a trim control within, but I have that adjusted to its minimum - I reckon the signal is still a little too hot for the unit.....I just need to reduce it a smidgen feeding into this unit.
If I'm reading this right - you're all saying that the load shouldn't be as low as 600 ohms - therefore a higher load is better?
My question is - at what point higher up the scale should I choose as a bare minumum (load wise) for a TL07x?
PS. This actual audio signal won't be heard - the output I show actually feeds a device that anaylyses the frequency/level content. The device I mention comes with a software editor interface which has a trim control within, but I have that adjusted to its minimum - I reckon the signal is still a little too hot for the unit.....I just need to reduce it a smidgen feeding into this unit.
Hank, there is no right or wrong answer here. MAXIMUM PEAK OUTPUT VOLTAGE vs LOAD RESISTANCE in Figure 9 of the TL07X datasheet says that a 600 ohm load can be driven to a maximum of 10V peak.
The issue is not whether the op-amp will work, but how distorted its signal will be when driving that load at the signal level you are using. You said the signal would be 2V peak, I think, so it will work fine, it just depends how important the distortion performance of the circuit is. I cannot hope to estimate what impact 600 ohms will have...it may be completely insignificant. So you will have to try it and see.
What other flexibility have you got? Can you attenuate at the input as has been suggested. That's the easiest thing to do. Or can you, say, live with a 1k output Z instead of 100 ohms?
The issue is not whether the op-amp will work, but how distorted its signal will be when driving that load at the signal level you are using. You said the signal would be 2V peak, I think, so it will work fine, it just depends how important the distortion performance of the circuit is. I cannot hope to estimate what impact 600 ohms will have...it may be completely insignificant. So you will have to try it and see.
What other flexibility have you got? Can you attenuate at the input as has been suggested. That's the easiest thing to do. Or can you, say, live with a 1k output Z instead of 100 ohms?
The pro audio company I worked for used to say that 2k was the minimum impedance to put on a TL072.
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