sahib
Well-known member
I think Ian intends to power it from the heater supply.In conditions when you have a limited power supply of 12V (how did you get + 20dBu of possible output voltage?) ...
I think Ian intends to power it from the heater supply.In conditions when you have a limited power supply of 12V (how did you get + 20dBu of possible output voltage?) ...
The +20dBu is from the output transformer of the tube stage. The semiconductor headphones amp will not be powered from the HT but from the 12V heater supplyIn conditions when you have a limited power supply of 12V (how did you get + 20dBu of possible output voltage?)
That looks to be just the job. It even provides balanced inputs.I would suggest using rail to rail headphone opamp chips like ct
https://www.ti.com/lit/ds/symlink/o...89841&ref_url=https%3A%2F%2Fwww.google.com%2F
No it doesn't. It's just an op amp. Figure 49 is just a typical balanced input arrangement.I would suggest using rail to rail headphone opamp chips like OPA1688.
That looks to be just the job. It even provides balanced inputs.
Indeed but most of the other op amp based topologies I looked at would have required another op amp to do thatNo it doesn't. It's just an op amp. Figure 49 is just a typical balanced input arrangement.
It seems to me there must be some significant difference between driving a 600 ohm load (5532) and a 32 ohm load (1688) - of course it is current drive capability. 1V rms into 32 ohms is 30mW and also 30mA. 30mW into 600 ohms is just 7mA. 5532 at maximum swing into 600 ohms (+-15V supply) is only 15mA. I am by no means an op amp expert but it seems to me that high currents directly into low impedances must lead to inefficiency in the class B output stage which in turn leads to potential thermal problems. Staying with the 1V into 32 ohms example the pp voltage is 2.828V. If I have a 12V rail the output transistors must dissipate (12 -2.828) x 30 milliwatts = 275mW. To be honest I would not be at all happy about dissipating that much power in an IC. I think I would rather use a discrete complementary pair tacked on the end of an op amp.There's nothing wildly different about OPA1688 specific to headphones. It looks like they just added a bunch of stuff to the datasheet about driving headphone loads. They just characterized using the op amps natural current limiting to handle the issue of different loads. But the data is convincing. Clearly OPA1688 would make a very good headphone amplifier that will limit current as desired.
Fortunately I don't plan on using dc couplingBut for a single supply you still have the issue of DC coupling.
This might help. They have a newer model as well.Looking for an off the shelf headphones amp PCB with balanced inputs and powered by a single 12V rail that I can integrate into a mixer. Any suggestions?
Cheers
Ian
I think I have to challenge that. If you put an SOIC-8 on a 2-3 square inch area of copper on top and below stitched together with lots of vias, that makes a nice heat sink. Probably better than just using bulker transistor packages. Certainly it could handle a 1/4 Wp.If I have a 12V rail the output transistors must dissipate (12 -2.828) x 30 milliwatts = 275mW. To be honest I would not be at all happy about dissipating that much power in an IC. I think I would rather use a discrete complementary pair tacked on the end of an op amp.
I had considered the latter but I try very hard to keep poer supplies outside of my mixers!While possibly 'not cricket' you could consider a small DC-DC convertor (possibly 3 Watt module) and get yourself a symmetrical plus and minus 12 /15 Volts or even a cheap miniature mains transformer with dual 12 Volt secondaries and use it as a voltage doubler (and chop off the old mains pins!).
Paralleled 5532s seems to be a recurring theme. According to my calculator, the 15 ohm emitter resistor drops 0,65V at around 40 something mA. Three in parallel would be interesting.Paralleling 2 or more sections of NE5532 (using the scheme shown on the earlier post with perhaps 200 Ohm output resistors for each op amp section used so perhaps 2 op amps per channel. One section as balanced input then 3 'halves' paralleled.
A single 5532 section is getting into clipping (current limit) driving 0dBu into 33 Ohms I happen to remember.
Hmmmm, I don't doubt that with that amount of copper it would handle 250mW but that is a lot of real estate that cannot be used for tracking.I think I have to challenge that. If you put an SOIC-8 on a 2-3 square inch area of copper on top and below stitched together with lots of vias, that makes a nice heat sink. Probably better than just using bulker transistor packages. Certainly it could handle a 1/4 Wp.
As Bo mentioned, any opamp can offer differential inputs and deliver an unbalanced output.Indeed but most of the other op amp based topologies I looked at would have required another op amp to do that
Neither would I, but it seems TI are not shy about it, although they restrict the output power to 30mW into 16R (the recommended max power for earbuds), which is about 5mA.To be honest I would not be at all happy about dissipating that much power in an IC.
That is a much simpler alternative and is known to work quite well. However, I'm not sure it exists in kit form.I think I would rather use a discrete complementary pair tacked on the end of an op amp.
How do you intend to do that? Are you using an smps with a virtual ground?Fortunately I don't plan on using dc coupling
Why would he need an SMPS?How do you intend to do that? Are you using an smps with a virtual ground?
Pretty close to what I was plannig.Why would he need an SMPS?
Seems to me a simple voltage divider / cap virtual ground would work perfectly fine. All of the output current is coming or going through the load. Even a non-inverting with gain wouldn't budge Vg. So OPA1688 with voltage divider / cap virtual ground into two 1000uF 10V caps and you're done. With caps you don't really have to worry about startup. Easy. Done.
I am not worried about CMRR in the connection to the phones amp input so there is no need for a quad pot. A single pot between hot and cold of each channel will suffice.As Bo mentioned, any opamp can offer differential inputs and deliver an unbalanced output.
However, the final arrangement should consider the nature of the source (bal /unbal) and the nature and placement of the volume control.
Unless you consider a quad volume pot or monitoring in mono, I believe you will separate the debal function and the power amplification.
So you're monitoring unbalanced sources?I am not worried about CMRR in the connection to the phones amp input so there is no need for a quad pot. A single pot between hot and cold of each channel will suffice.
I asked because I don't know how you're going to produce 12V from a supposed 6.3Vac source.Pretty close to what I was plannig.
Does it mean that the headphone will be connected between opamp's output and virtual ground point which sits at V/2 and is made with two resistors and two big caps?Seems to me a simple voltage divider / cap virtual ground would work perfectly fine. All of the output current is coming or going through the load. Even a non-inverting with gain wouldn't budge Vg. So OPA1688 with voltage divider / cap virtual ground into two 1000uF 10V caps and you're done. With caps you don't really have to worry about startup. Easy. Done.
Ian uses 12V supply for heaters (connected in series).I asked because I don't know how you're going to produce 12V from a supposed 6.3Vac source.
No. If you have a cap in there you can source / sink from / to the regular host 0V (which is the negative rail from the prospective of the op amp). When sourcing current it's 12V > output > coupling cap > voice coil > host 0V but when sinking current it's voice coil > couping cap > output > host 0V. So it's a little odd in that the coupling cap is actually what sinks current from the voice coil on negative excursions. But I don't think there is any consequence for doing that. Whatever works.Does it mean that the headphone will be connected between opamp's output and virtual ground point which sits at V/2 and is made with two resistors and two big caps?
https://angryaudio.com/headphonegizmo/I also came across a thing called the 47 amp which seems to be an old headwize project using both op amps of a NE5532 in a current boosted configuration. But I doubt this will drive a 32 ohm headphone.
Cheers
This is probably not what you are looking for, but it still might be of some interest:
Pretty contradictory with what Ian wrote: "Fortunately I don't plan on using dc coupling"No. If you have a cap in there you can source / sink from / to the regular host 0V (which is the negative rail from the prospective of the op amp). When sourcing current it's 12V > output > coupling cap > voice coil > host 0V but when sinking current it's voice coil > couping cap > output > host 0V. So it's a little odd in that the coupling cap is actually what sinks current from the voice coil on negative excursions.
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