Ok sounds good, I'll leave the 470 uF caps where they are then. I'll use a good quality cap like Elna Silmic II or Nichicon UKZ there.
Which capacitor do you mean by C20? Sorry if the schematic wasn't clear before, here's a higher resolution image if that helps:
Would 1-2 uF caps work for this? I calculated that with the input resistor at 100K and a 2 uF cap the Fc would be 0.796 Hz. I'm not sure how the series resistance of the potentiometers would affect that but it would be nice to use a lower capacitance if possible to allow the use of film caps
I will add DC blocking caps between the wiper and op-amp in the volume and balance sections, but is the third potentiometer you mention the bass pot in the tone control? If so would the DC blocking cap go between R13/C13 and the negative input of U7?
No it goes on the wiper as always - you must allow the opamp inputs to have DC bias current or it will latchup, you must not allow that bias current to flow wholely or partially through the wiper of a pot.
I see another potential issue which may be relevant as you seem to aim for max. quality: The Baxandall Tone control inverts the audio signal. In case of tone control bypass this inversion does not happen anymore. There are reports that inverted vs. non inverted audio signals sound slightly different which may or may not bother you... But be aware that when tone controls are in neutral position and you switch between Tone and bypass, the sound may differ more due to polarity inversion than due to one less OP stage in the path...
Greetings,
Winfried
Greetings,
Winfried
Alternatively there is also the LME49720 which is also DIP which looks to be pretty much the same as the LM4562.
I wonder if it has the same quality issues.
If you are looking for a better opamp in DIP you can also check the AD797 or the LT1028 but beware the LT1028 is not unity gain stable so you can't use it as a buffer.
You can also use SO8 to DIP Adapters to have a wider selection of opamps to choose from.
I wonder if it has the same quality issues.
If you are looking for a better opamp in DIP you can also check the AD797 or the LT1028 but beware the LT1028 is not unity gain stable so you can't use it as a buffer.
You can also use SO8 to DIP Adapters to have a wider selection of opamps to choose from.
My ears can't tell whether music starts at 0 or Pi as long as both channels are inverted or not ;-)
There have been double-blind tests done showing that there are people who can hear polarity, at least when listening to monaural recordings of asymmetrical waveforms over headphones or in acoustically dead rooms.
Greiner and Melton, "Observations on the audibility of acoustic polarity", Journal of the Audio Engineering Society, April 1994.
https://www.semanticscholar.org/pap...lton/3efed648dcd433f4441e9f5c48e4669a0f0a0ab4
https://www.aes.org/e-lib/browse.cfm?elib=6950
Greiner and Melton, "Observations on the audibility of acoustic polarity", Journal of the Audio Engineering Society, April 1994.
https://www.semanticscholar.org/pap...lton/3efed648dcd433f4441e9f5c48e4669a0f0a0ab4
https://www.aes.org/e-lib/browse.cfm?elib=6950
My old Preamp had some TLE2071 and after replacing them with some newer opamps I realized that the TLE2071 is actually a good sounding opamp eventhough the values in the datasheet don't look that good.
Using an opamp with a FET input stage might be a better choice if you have a high input impedance.
Using an opamp with a FET input stage might be a better choice if you have a high input impedance.
Dear Ralf,
it has proven to be risky to extrapolate to general validity from an individual subjective perception (which I am not doing 😉). My statement positioning as "potential issue" was done for the reason that it may have been overlooked and may or may not be a relevant concideration for the thread owner.
Greetings,
Winfried
palmtrees,
I designed an amplifier for you. Personally, I would do it that way. The concept is bullet proof, simple, cheap, and in terms of sound it surpasses 90% of all preamplifiers on the market. I recommend good Alps pots and the Alps Balance Pot. Then you don't lose 6dB of signal.
Last edited:
LM4562 has 10nA input bias current and 0.1mV DC offset. I don't see much reason for blocking caps on pots but YMMV.
What do you think happens when the bias current gets interrupted over a scratchy bit of the pot? The opamp output slams to the rail (+/-15V perhaps) on a sub-millisecond timescale with a massive crack that will make you jump! Only FET input opamps can handle this (since stray capacitance will limit the input voltage slewing with picoamp bias currents to a few volts/second rather than the kV/s or MV/s of a bipolar input).
With DC-blocking cap on the wiper the same scratchiness will just modulate the signal (i.e. annoying rustling, but won't throw your speaker cones across the room!)
Its common to see tone control sections use TL072's to avoid having to add DC-blockers to the pots, even in circuits where the other opamps are lower noise and lower distortion.
With DC-blocking cap on the wiper the same scratchiness will just modulate the signal (i.e. annoying rustling, but won't throw your speaker cones across the room!)
Its common to see tone control sections use TL072's to avoid having to add DC-blockers to the pots, even in circuits where the other opamps are lower noise and lower distortion.
Last edited:
In the circuits of posts #36 and #42, I think you would get a temporary shift of the output voltage of at most 5 mV with typical bias currents, 36 mV with worst-case bias currents, when the tone controls are on neutral.
The worst case is the volume potmeter. When its wiper gets disconnected, the 10 nA typ., 72 nA max. bias current of U3 has to flow through a 100 kohm resistor, causing a shift of at most 1 mV typ., 7.2 mV max. This shift is amplified almost 5 times by the next op-amp, U4.
The worst case is the volume potmeter. When its wiper gets disconnected, the 10 nA typ., 72 nA max. bias current of U3 has to flow through a 100 kohm resistor, causing a shift of at most 1 mV typ., 7.2 mV max. This shift is amplified almost 5 times by the next op-amp, U4.
Last edited:
Yes this circuit seems to have mitigated the problem reasonably well (some extra resistors added) for a low bias current bipolar, but in general its something that can bite you hard.
Yes the use of larger capacity value electrolytics than frequency necessary is to negate the distortion effects .
I would like to get the best quality possible out of this design, as my power amp is already pretty good and I don't want the pre-amp to limit the sound quality.
I definitely did not consider the phase inversion that would result from the tone control bypass, thanks for bringing that up. If I want to correct for phase inversion there, would a unity gain inverting op-amp in the bypass circuit be a good way to go?
I was thinking about trying different op-amps once the circuit is finalized and I get a breadboard prototype built. I've read that different op-amps have different sound qualities. I'll look into the LME49720 and AD797. My original plan was to build the pre-amp with the same op-amps for everything, but now I am starting to consider using different op-amps in the circuit that are fine tuned for their specific purpose. I still have lots of research to do in this area though.
I got the tone control section from the LM4562 datasheet but Doug Self has a similar circuit in Small Signal Audio Design. In the section on Baxandall tone controls he says the circuit assumes a FET op-amp and DC blocking caps should be added to minimize bias currents.
But what is a good value for the bass pot? The treble pot already has an 8 nF cap in series with the inverting input. Is that enough, or should a higher capacitance capacitor be added in series with C13?