This is an example of one of my applications, this is a simple stereo line stage which is powered by a 100mA dc to dc converter 5V in and +/-15V out. Given the compact size a standard PSU would not have fit, and USB was convenient and available. DC power out is filtered by a common mode 3 wire choke, and the DC is pretty clean (a couple of mV at 25kHz), but there is a bit of radiation into surrounding wiring so I am going to try adding some shielding around the converter and chokes.
These were mailed out last week, please let me know when you have received them. 🙂
I mailed out a couple of sets of blank boards to participants in this thread, unfortunately I haven't heard anything since.
As you know, I've designed four pretty good projects in the past year and a half and documented them extensively. A few people seemed interested in building them, and some even got started, then silence. It's strange. I do know it's easy to get excited about a project and to order parts, then get a sinking feeling when you start thinking about all the worked involved in completing them.
It seems par for the course these days. For me the main challenge is finding the right parts for my projects, the rest of it is just fun.
LOL, it would be difficult or impossible for me to duplicate my projects today if I didn't have a stash of spare parts.
Never attribute to incompetence that which is adequately explained by malice.
Henry Pasternack
Never attribute to incompetence that which is adequately explained by malice.
Henry Pasternack
I noticed a couple things in your schematic that may be relevant to clipping issues.
1. No Baker clamp on the VAS. A Baker clamp keeps away rail sticking and other strange behavior coming out of clipping. For a Darlington VAS, an ordinary diode is typical. Some complain about 1N4148 non-linear capacitance? You can clamp just the second transistor as long as the current in the first is limited. Even a 1N4148 on a single transistor or two on a Darlington can clean up the VAS enough to avoid problems. Without a Baker clamp or current limit on the Darlington Q1, you can get phase inversion as the CB becomes forward biased, especially with an emitter degen resistor.
2. On the other side, the LTP and VAS CCSs share a bias source. This can be a problem at clipping where there is ~no VAS CCS collector current, so the VAS CCS draws a lot of current from the base and loads the bias, feedback to the LTP CCS. Usual solution is a base resistor on the VAS CCS to limit the current it can take from the bias cct, or a separate bias cct.
2a. And It may be useful to start the LTP CCS before the VAS CCS to reduce on-off thumps. You can do that with 3x VBE for the LTP CCS and 2x for the VAS CCS.
I discovered that JFET as the LTP can cause a problem, especially with lower supply voltages. If the JFET vto is to high it subtracts from the common mode range, a problem for 0dB buffers. I note the JFET TL071 op-amps have cmr limits a couple volts from the rail, indicating the input JFET vto are not more than ~0.7V, ie vto + 2xVBE ~=2V.
In a circuit where I used a current mirror as an inverter, I found that a Wilson CM had less distortion, but when I tried it in an op-amp cct, that was not true ?? and there are minimum voltage drop issues.
Hope this is useful.
1. No Baker clamp on the VAS. A Baker clamp keeps away rail sticking and other strange behavior coming out of clipping. For a Darlington VAS, an ordinary diode is typical. Some complain about 1N4148 non-linear capacitance? You can clamp just the second transistor as long as the current in the first is limited. Even a 1N4148 on a single transistor or two on a Darlington can clean up the VAS enough to avoid problems. Without a Baker clamp or current limit on the Darlington Q1, you can get phase inversion as the CB becomes forward biased, especially with an emitter degen resistor.
2. On the other side, the LTP and VAS CCSs share a bias source. This can be a problem at clipping where there is ~no VAS CCS collector current, so the VAS CCS draws a lot of current from the base and loads the bias, feedback to the LTP CCS. Usual solution is a base resistor on the VAS CCS to limit the current it can take from the bias cct, or a separate bias cct.
2a. And It may be useful to start the LTP CCS before the VAS CCS to reduce on-off thumps. You can do that with 3x VBE for the LTP CCS and 2x for the VAS CCS.
I discovered that JFET as the LTP can cause a problem, especially with lower supply voltages. If the JFET vto is to high it subtracts from the common mode range, a problem for 0dB buffers. I note the JFET TL071 op-amps have cmr limits a couple volts from the rail, indicating the input JFET vto are not more than ~0.7V, ie vto + 2xVBE ~=2V.
In a circuit where I used a current mirror as an inverter, I found that a Wilson CM had less distortion, but when I tried it in an op-amp cct, that was not true ?? and there are minimum voltage drop issues.
Hope this is useful.
Actually it is useful, and thank you for the thoughtful post. There is a lot of good information in it. The intended purpose is for a discrete tape amplifier, clipping should never be a problem, but should I decide to use them elsewhere it could well be. At some point I am going to pick up where I left off.
I actually have the feeling that this design is already a bit too complex. Interesting comments on the Baker Clamp. Henry has had some interesting observations on baker clamp behaviors which lead me to leave it out for this application.
The CMR of the input stage does not extend very close to either rail, I no longer remember the range as it's been quite a while since I worked on any of these designs. They were not designed for unity gain, although as long as you stay below 20Vpp on the inputs you should be OK. In my application (and ones for which they were intended) the minimum required gains are at least 6dB.
The maximum output on +/-15V rails is about 9Vrms which is more than sufficient for my original application which is a tape amplifier..
This is what all of these different op-amps were destined for use in. This is my last R2R tape amp project, supports NAB and IEC EQ at 7.5 and 15ips.
I actually have the feeling that this design is already a bit too complex. Interesting comments on the Baker Clamp. Henry has had some interesting observations on baker clamp behaviors which lead me to leave it out for this application.
The CMR of the input stage does not extend very close to either rail, I no longer remember the range as it's been quite a while since I worked on any of these designs. They were not designed for unity gain, although as long as you stay below 20Vpp on the inputs you should be OK. In my application (and ones for which they were intended) the minimum required gains are at least 6dB.
The maximum output on +/-15V rails is about 9Vrms which is more than sufficient for my original application which is a tape amplifier..
This is what all of these different op-amps were destined for use in. This is my last R2R tape amp project, supports NAB and IEC EQ at 7.5 and 15ips.
FWIW, I had the sense in a headphone amplifier I designed that Baker clamp diodes across the VAS caused some harshness when using two-pole compensation. I did not hear a problem with Miller compensation, where the diodes are directly in parallel with the compensation capacitor. I theorized that the nonlinear junction capacitance was the problem in the TPC case, but have no way to prove it. I ended up ditching TPC in this circuit and leaving the diodes in, and was very happy with the performance, technical and audible.
I'd rather not debate it, though. Consider my report a data point, worth what you paid for it.
YMMV.
-Henry
Here is a circuit I came up with that mitigates the issue of non-linear diode capacitance when used for a Baker clamp. The idea is not to connect the diode directly to the EF input, but through a buffer transistor (Q15) instead, which is biased by a LED to slightly reverse-bias its BE junction during normal operation, so it stays out of the circuit. During clipping, the VAS will turn on the diode, which will turn on the buffer and limit the VAS input like the regular Baker clamp would.
Attachments
Hi Kevin and others,
Appreciate the work and thought you al have put into this project. May try and incorporate the design into one of my Stellavox's.
Was going to ask you for a pair of PC boards but see they incorporate SMD's - which is beyond my assembly desires/capabilities.
Take care all!
Charles
Appreciate the work and thought you al have put into this project. May try and incorporate the design into one of my Stellavox's.
Was going to ask you for a pair of PC boards but see they incorporate SMD's - which is beyond my assembly desires/capabilities.
Take care all!
Charles