hey guys i was just wondering if soemone could help me add a volume control to this circuit?
thank you for the help!
cheers
thank you for the help!
cheers
Add the volume pot on the input of the amplifier.
A 10 kOhm pot should be perfect. Make sure you use a pot with audio taper (logarithmic pot). You may need an op-amp buffer between the volume pot and the power amp as the input impedance of the amp is about 20 kOhm, which will load the volume pot quite a bit.
~Tom
A 10 kOhm pot should be perfect. Make sure you use a pot with audio taper (logarithmic pot). You may need an op-amp buffer between the volume pot and the power amp as the input impedance of the amp is about 20 kOhm, which will load the volume pot quite a bit.
~Tom
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Add the volume pot on the input of the amplifier.
A 10 kOhm pot should be perfect. Make sure you use a pot with audio taper (logarithmic pot). You may need an op-amp buffer between the volume pot and the power amp as the input impedance of the amp is about 20 kOhm, which will load the volume pot quite a bit.
~Tom
thank you for your reply and help! very much appreciate it! would it be possibly to show me the steps/equation for solving Zin?
cheers
Zin depends on the pot setting. At minimum, it is 10k, and at maximum it is 10k in parallel with 20k, or 6.66k.
The equation depends on adjustment of the pot, though.
thank for the help guys!
can i use a 741 general op amp as the buffer (unity gain)
also, do i need to supply the buffer with Vcc?
thanks again
can i use a 741 general op amp as the buffer (unity gain)
also, do i need to supply the buffer with Vcc?
thanks again
Your vol pot needs a Buffer if it can't drive the cable and the input of the Receiver.
If the cable is 50mm long then a 10k vol pot can probably drive the this combination if the Receiver has an input impedance of >25k and does not have an enormous RF attenuation capacitor.
A Buffer has to handle AC signals.
That usually means it needs a Dual Polarity Power Supply.
You can use a single polarity supply if the input to the Buffer and the output from the Buffer are AC coupled.
A Buffer can be as simple as a single transistor used as an emitter/source follower, or you can use an opamp that performs well when set to unity gain.
If the cable is 50mm long then a 10k vol pot can probably drive the this combination if the Receiver has an input impedance of >25k and does not have an enormous RF attenuation capacitor.
A Buffer has to handle AC signals.
That usually means it needs a Dual Polarity Power Supply.
You can use a single polarity supply if the input to the Buffer and the output from the Buffer are AC coupled.
A Buffer can be as simple as a single transistor used as an emitter/source follower, or you can use an opamp that performs well when set to unity gain.
Decibel Dungeon has detailed articles on all the various buffers that Nick (Nuuk on here) has used and his opinions on them.
http://www.decdun.me.uk/gaincloneindex.html
I've used the Uno active pre with both LM3875s and 1875s and like it very much.
Hope it helps
John
http://www.decdun.me.uk/gaincloneindex.html
I've used the Uno active pre with both LM3875s and 1875s and like it very much.
Hope it helps
John
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would this work?
An externally hosted image should be here but it was not working when we last tested it.
Hi,
A buffer is not needed, generally. Also note the typical value
of RFa is not 20K, it is much higher, and sets amplifier gain.
RFa could be varied for the volume control, as the device is
unity gain stable. Probably need 200K to 470K audio taper.
rgds, sreten.
A buffer is not needed, generally. Also note the typical value
of RFa is not 20K, it is much higher, and sets amplifier gain.
RFa could be varied for the volume control, as the device is
unity gain stable. Probably need 200K to 470K audio taper.
rgds, sreten.
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I presume we're looking at a part operating from only +5V or somesuch, a gain of -1 seems sufficient for that - especially since we're getting 6 dB extra from the BTL arrangement, so it really is -2 in total.
A 10k volume pot with nothing else should work splendidly. With a low-impedance source output connected, its output impedance never goes beyond 2.5 kOhms, so the effect of the 20k input impedance is going to be fairly negligible.
A 10k volume pot with nothing else should work splendidly. With a low-impedance source output connected, its output impedance never goes beyond 2.5 kOhms, so the effect of the 20k input impedance is going to be fairly negligible.
okay. so we have conflicting ideas. some say use a buffer, some say its not needed. which is correct?
if i need a buffer, can i make it with any of the follow
2N5458 - Gen. JFET
2N3904 NPN BJT
2N5306 - NPN Darlington
LM348N - quad op amp
UA741 - op amp
if i need a buffer, can i make it with any of the follow
2N5458 - Gen. JFET
2N3904 NPN BJT
2N5306 - NPN Darlington
LM348N - quad op amp
UA741 - op amp
with no buffer the tone of the stuff might change with volume.
in most cases the change in tone can not even be recognised, in other cases it is welcome.
the geek solution is:
Buffer -> volume pot -> buffer -> amplifier.
it lets the source see a high impedance,
and isolates any effects of the volume pot besides what you want from the poweramp.
if you ask me, just add the pot and that's it.
in most cases the change in tone can not even be recognised, in other cases it is welcome.
the geek solution is:
Buffer -> volume pot -> buffer -> amplifier.
it lets the source see a high impedance,
and isolates any effects of the volume pot besides what you want from the poweramp.
if you ask me, just add the pot and that's it.
Hi,
Read the datasheet, the right / simplest answer is obvious.
Clue : I'm not wrong, and most are making mis-assumptions.
rgds, sreten.
Real numbers depend on the supply voltage and nominal
maximum input level, its all described in the datasheet.
Except for making RFa variable as a volume control.
Technically varying RFa is the most superior approach.
Read the datasheet, the right / simplest answer is obvious.
Clue : I'm not wrong, and most are making mis-assumptions.
rgds, sreten.
Real numbers depend on the supply voltage and nominal
maximum input level, its all described in the datasheet.
Except for making RFa variable as a volume control.
Technically varying RFa is the most superior approach.
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If i were to add a volume control as RFa, what value of pot (and type) would be needed?
Pin 1 of the chip amp is virtual ground. Hence, the input impedance at the "AUDIO INPUT" connector in the circuit in Post #1 is: Zin(s) = Rin + 1/(sCin), where s is complex frequency. At DC this becomes Zin = Rin.
An op-amp text such as Franco will be helpful to figure out the terms...
~Tom
thank for the help guys!
can i use a 741 general op amp as the buffer (unity gain)
The 741 is not a good choice for audio due to it's rather severe slew limitations. I suggest something like the NE5532 if you're Old Skool, LME49710 if you want modern performance, or OPA134 if you're contemporary modern with an Old Skool bent... 🙂
~Tom
Hi, see post #10, rgds, sreten.
Pot value determines maximum gain.
The rest is all in the datasheet.
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.
Jumping in, maybe the circuit posted below will be of interest. It's mostly a shameless rip from D. Self. Also the pot suggested by tomchr hooked to a buffer.
The advantage of a buffer is very high input impedance, and very low output impedance. This results in absolute separation between circuit stages preceding and following the buffer, preventing interaction between different components.
CIRCUIT NOTES:
Rvol is, of course, the volume control. Together with R2 it sets the circuit input impedance (Zin). Zin is the parallel value of Rvol and R2 (Rvol || R2), here about 25k.
R1 and C1 together form a low-pass filter that shunts ultrasonic frequencies to ground. This takes a bit of load off the op amp (because it doesn't have to amplify those frequencies), and helps prevent oscillation.
C2 and R2 together form a high-pass filter, which here rolls off frequencies below about 20Hz. R2 also sets Zin as previously noted.
R3 is a "just in case" resistor that can help prevent oscillation under some conditions.
R4 is commonly included in circuits, it prevents problems with capacitance in the output line.
Building Notes: Keep components as close together as your skill level reasonably allows, but no need to obsess. Importantly, keep audio signal conductors away from power supply conductors. Do not allow the audio signal to mix with power supply voltages except at circuit ground.
RadioShack's 25.2 volt, 450ma transformer #2264950 goes for $8, at this writing on sale for $6. Also convenient is RadioShack's perfboard #2266451. For other components I've had good results using this eBay seller: 1 4W 5, Ceramic Disc Capacitors items in Thai Shine store on eBay! Disclosure: I have no association with any of these people except to send them money sometimes.
.
Jumping in, maybe the circuit posted below will be of interest. It's mostly a shameless rip from D. Self. Also the pot suggested by tomchr hooked to a buffer.
The advantage of a buffer is very high input impedance, and very low output impedance. This results in absolute separation between circuit stages preceding and following the buffer, preventing interaction between different components.
CIRCUIT NOTES:
Rvol is, of course, the volume control. Together with R2 it sets the circuit input impedance (Zin). Zin is the parallel value of Rvol and R2 (Rvol || R2), here about 25k.
R1 and C1 together form a low-pass filter that shunts ultrasonic frequencies to ground. This takes a bit of load off the op amp (because it doesn't have to amplify those frequencies), and helps prevent oscillation.
C2 and R2 together form a high-pass filter, which here rolls off frequencies below about 20Hz. R2 also sets Zin as previously noted.
R3 is a "just in case" resistor that can help prevent oscillation under some conditions.
R4 is commonly included in circuits, it prevents problems with capacitance in the output line.
Building Notes: Keep components as close together as your skill level reasonably allows, but no need to obsess. Importantly, keep audio signal conductors away from power supply conductors. Do not allow the audio signal to mix with power supply voltages except at circuit ground.
RadioShack's 25.2 volt, 450ma transformer #2264950 goes for $8, at this writing on sale for $6. Also convenient is RadioShack's perfboard #2266451. For other components I've had good results using this eBay seller: 1 4W 5, Ceramic Disc Capacitors items in Thai Shine store on eBay! Disclosure: I have no association with any of these people except to send them money sometimes.
.
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Hi,
Its a cheap chip, 3W stereo into 4 ohms or 6W BTL into 8 ohms.
Your all massively overcomplicating something essentially simple.
That it is unity gain stable basically tells you it is not really hifi,
but also tells you there is a very simple classic solution, that
is the same as an active shunt feedback volume control.
rgds, sreten.
http://www.ti.com/lit/ds/symlink/lm4950.pdf
Its a cheap chip, 3W stereo into 4 ohms or 6W BTL into 8 ohms.
Your all massively overcomplicating something essentially simple.
That it is unity gain stable basically tells you it is not really hifi,
but also tells you there is a very simple classic solution, that
is the same as an active shunt feedback volume control.
rgds, sreten.
http://www.ti.com/lit/ds/symlink/lm4950.pdf
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