Ian,
we have repeated the simulations.
As in specified in the previous mail, the results of the tests are reported in the yellow boxes (mV) of the schematics. In the green boxes have been reported the -3dB Hz (these last data are presumed and not yet been simulated).
We again included the updated version of the circuits with the relative data. As you can see, we applied all your suggestions…with only two little variations (the substitution of 22K pot with a 47K one and the elimination of the bypass cathode capacitor).
1) In fact, with the exception of vocal section, in the remaining 3 output stages the 22K gain control you proposed has been changed with a 47K, in order to reduce the capacitance of the upstream coupling capacitors (and so…dimensions and cost too).
Is it still correct ?
2) It seems that the signals are so higher that no cathode bypass capacitor is necessary. However, with this configuration … does the vocal effect bus amplifier (with lower output current and higher impedance) still adequate to drive the “send” output with 1-2 mt of cable and the downstream reverb device ?
3) The schematics utilized for the simulations are reported as attached pdf files. We hope they are now correct.
4) A 100k resistor has been proposed to add on the secondary of the output transformer. What do you think?
5) In view of the relatively high output values (with the worst condition the outputs are 11-18V) … does the 600:600 output transformer still represent the best solution or … a stepdown one could better fulfill the role?
Attention … The measures on the 4 output stages have been performed downstream the output capacitor … and not on the secondary of the output transformer.
Ciao
Antonio e Francesco
we have repeated the simulations.
As in specified in the previous mail, the results of the tests are reported in the yellow boxes (mV) of the schematics. In the green boxes have been reported the -3dB Hz (these last data are presumed and not yet been simulated).
We again included the updated version of the circuits with the relative data. As you can see, we applied all your suggestions…with only two little variations (the substitution of 22K pot with a 47K one and the elimination of the bypass cathode capacitor).
1) In fact, with the exception of vocal section, in the remaining 3 output stages the 22K gain control you proposed has been changed with a 47K, in order to reduce the capacitance of the upstream coupling capacitors (and so…dimensions and cost too).
Is it still correct ?
2) It seems that the signals are so higher that no cathode bypass capacitor is necessary. However, with this configuration … does the vocal effect bus amplifier (with lower output current and higher impedance) still adequate to drive the “send” output with 1-2 mt of cable and the downstream reverb device ?
3) The schematics utilized for the simulations are reported as attached pdf files. We hope they are now correct.
4) A 100k resistor has been proposed to add on the secondary of the output transformer. What do you think?
5) In view of the relatively high output values (with the worst condition the outputs are 11-18V) … does the 600:600 output transformer still represent the best solution or … a stepdown one could better fulfill the role?
Attention … The measures on the 4 output stages have been performed downstream the output capacitor … and not on the secondary of the output transformer.
Ciao
Antonio e Francesco
OK, I have had a quick look and I still think I see some errors. For instance, the all the channels on bass case still has all the 1Meg resistors in circuit. Although these are no longer shorted to ground by S6, they are connected to ground by the low output impedance of the driving SRPP stage. A reasonable guess for the SRPP output impedance would be 1500 ohms. This is how passive mixing works. It relies on there always being a low impedance path to ground across those 1Meg resistors either via S6 or the output impedance of the prior amplifier stage.
Also, I notice that the preset pot and master fader between the bus amp and the output amp are set at 100%. These need to be set to reflect a 10dB loss in the master fader and about 9dB in the preset pot as discussed in my 13th January post.
Cheers
Ian
Ian,
we have corrected the resistor values in the simulations and have (again) repeated them.
Now we have understood the theory of passive mixer (we hope).
Pay attention at the mV values reported in the yellow boxes: they are expressed as Vpk (N.B. Vpk and not Vpkpk nor Vrms) !!!
In the green boxes have been reported the -3dB Hz (these last data are presumed and not yet been simulated).
We remember you that only two little changes have been made (the substitution of 22K pot with a 47K one for the trim pot and the elimination of the bypass cathode capacitor).
Now the output values are too little in some cases (but we have not added the coupling cathode resistors).
Without the -9dB on the trim pot upstream master fader the output values are still good (also with -10dB in the master fader).
1) The first question is: is necessary the extra -9dB on the trim pot (output gain - Vr14, Vr16, Vr18, Vr20) ? ... why is not sufficient the 10dB extragain on master fader?
Other questions...
2) We have changed your 22K trim pot with a 47K one in order to reduce the capacitance of the upstream coupling capacitors (and so…dimensions and cost too). Is it still correct ?
3) If we could exclude the extra gain of 9 dB on trim pot... the cathode resistor seems to be not necessary. In this case, does the vocal effect bus amplifier (with lower output current and higher impedance) still adequate to drive the “send” output with 1-2 mt of cable and the downstream reverb device ?
4) The coupling capacitor in the bus amplifiers of instr is now 6.8 uF (due to the lowest possible load on these buses ... about 1.4K). With this value (and that load) we calculated a -3dB at 17Hz... but we presume a no flat curve untill about 150-160 Hz. What do you think about the value of this capacitor?
5) A 100k resistor has been proposed to add on the secondary of the output transformer. What do you think?
Attention … The measures on the 4 output stages have been performed downstream the output capacitor … and not on the secondary of the output transformer.
Ciao
Antonio e Francesco
we have corrected the resistor values in the simulations and have (again) repeated them.
Now we have understood the theory of passive mixer (we hope).
Pay attention at the mV values reported in the yellow boxes: they are expressed as Vpk (N.B. Vpk and not Vpkpk nor Vrms) !!!
In the green boxes have been reported the -3dB Hz (these last data are presumed and not yet been simulated).
We remember you that only two little changes have been made (the substitution of 22K pot with a 47K one for the trim pot and the elimination of the bypass cathode capacitor).
Now the output values are too little in some cases (but we have not added the coupling cathode resistors).
Without the -9dB on the trim pot upstream master fader the output values are still good (also with -10dB in the master fader).
1) The first question is: is necessary the extra -9dB on the trim pot (output gain - Vr14, Vr16, Vr18, Vr20) ? ... why is not sufficient the 10dB extragain on master fader?
Other questions...
2) We have changed your 22K trim pot with a 47K one in order to reduce the capacitance of the upstream coupling capacitors (and so…dimensions and cost too). Is it still correct ?
3) If we could exclude the extra gain of 9 dB on trim pot... the cathode resistor seems to be not necessary. In this case, does the vocal effect bus amplifier (with lower output current and higher impedance) still adequate to drive the “send” output with 1-2 mt of cable and the downstream reverb device ?
4) The coupling capacitor in the bus amplifiers of instr is now 6.8 uF (due to the lowest possible load on these buses ... about 1.4K). With this value (and that load) we calculated a -3dB at 17Hz... but we presume a no flat curve untill about 150-160 Hz. What do you think about the value of this capacitor?
5) A 100k resistor has been proposed to add on the secondary of the output transformer. What do you think?
Attention … The measures on the 4 output stages have been performed downstream the output capacitor … and not on the secondary of the output transformer.
Ciao
Antonio e Francesco
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If you do not lose 9dB in the trim pot you have to lose it in the master fader so the normal setting for this would be at -19dB which is close to the half way point on a normal log pot. There is no technical reason why you should not do this but you need to ensure that the operators of the mixer know where the normal position is for this fader. It is normal practice to have no more than 10dB extra gain available on a master fader.
Other questions...
Yes, that is OK.
That should be OK over that short distance.
I do not understand this. The load on the output of the bus amplifiers is only the trim pot and the master fader. What makes you think it is 1.4K?
This should be connected across the primary of the transformer so as to provide a charge/discharge path for the capacitor if the transformer primary is not connected e.g. for testing.
LTspice is a very good program. I use it myself a lot. If you select ac analysis and the start and end frequencies to say 1Hz and 1MegHz, then set your ac source to 1mV, it will plot the frequency response and gain for you so finding the -3dB points is easier.
Cheers
Ian
You are right. It was related to C13 (V4 coupling capacitor … of the preamplifier board)…and, due to an erroneous calculation of the load (1.4K), it resulted to be of high capacitance (6.8uF). This happened because of an erroneous connection in the simulation schematic. After the correction, the worst load case is 21.7K, so C13 has been chosen of 2.2uF (with -3dB at 3Hz).
As a result of the correction, most of the values of instr are now higher.
Also the coupling capacitor of buses amplifiers was wrong. Now it has been corrected and adapted to 40K load for vocal (22K-> 22K||100K=40K) and 32K for instr and bass (47K||100K=32K).
This concept is not clear to us (and we beg your pardon for our ignorance
).We know that the master fader is usually put on "0" (for an output of +4dBU) and it has a possibility of run toward +10dB or -infinite dB. So the further +9 dB are obscure to us.
Ciao
Antonio e Francesco
This concept is not clear to us (and we beg your pardon for our ignorance
We know that the master fader is usually put on "0" (for an output of +4dBU) and it has a possibility of run toward +10dB or -infinite dB. So the further +9 dB are obscure to us.
Ciao
Antonio e Francesco
The output of the bus amplifiers is close to +4dBu. The output amplifiers have a gain of about 19dB. For the output of the output amplifier to also be close to +4dBu we therefore need to attenuate the signal by a total of 19dB. I have suggested we have 9dB loss in the trim pot and 10dB loss in the master fader.
Cheers
Ian
Now it seems clear 
. We have to regulate the trim pot in order to have +4dBU in output when the master fader is on the "0"line... but, at the same time, it (fader master) must be have the possibility of an extra 10dB gain.
Ian do you agree with the cut-off values of the output capacitors?
Many thanks for your patience
Anotnio
. We have to regulate the trim pot in order to have +4dBU in output when the master fader is on the "0"line... but, at the same time, it (fader master) must be have the possibility of an extra 10dB gain. Ian do you agree with the cut-off values of the output capacitors?
Many thanks for your patience
Anotnio
Now it seems clear
That is 100% correct!
I think the capacitor for the vocal send need to be increased because this could be driving a 10K bridging input of an external device. This should probably be increased to 2.2uF. I notice all the others are 2.2uF except the vocal clean. I would suggest you make them all 2.2uF for simplicity.
Cheers
Ian
I would say that Lexicon is probably the most well respected manufacturer of outboard reverb. They have a range of products from the budget priced MX200 up to very expensive professional units like their PCM96 that are popular in studios today.
Cheers
Ian
Dear Ian,
I have discussed with Francesco about mixer requirements and it seems that it remains to add some sections …before organize the layout of the PCBs. In particular, it is necessary to include
1) An output (driving 30 mt of cables) for monitors
2) A mic amplifier for the monitor mic (through which the technician of the mixer can talk to musicians).
3) A headphone output
4) Vu meters for the 4 output stages
5) Output recording.
As to point 1, 2, and 3 we think that high audio quality is not necessary in these sections, so a single chip solution could be the best.
1) As to the first one (monitor outputs), we have thought to THAT IC, a nice single chip solution for the balanced line driver THAT Corporation 1606/1646 OutSmarts Balanced Line Driver ICs
- What do you think about it ? Have you other solutions?
- Could we directly connect the 4 outputs of TL082 to 4 THAT 1646 ICs (without capacitors) ?
2) As to monitor mic we think that a single chip solution for both should be the best.
- Do you suggest a particular chip?
- How to connect the output of chip with the four monitor channels ? Is a 22K R enough for a distribution of the signal ? … or we have to put a R to ground for each channel too?
\\\\\\\\\\\\\\\\\|--22K -> monitor 1 (to R35)
Chip---->------ |--22K -> monitor 2 (to R36)
\\\\\\\\\\\\\\\\\|--22K -> monitor 3 (to R37)
\\\\\\\\\\\\\\\\\|--22K -> monitor 4 (to R38)
3) As to headphone output we think that a single chip solution for both should be the best.
- Have you some suggestion for it ?
- Are correct the connection reported in the schematic?
4) For the master fader we think that a great display Vu meters are necessary.
- Which Vu meters do you suggest? Led type or analog type?
5) As to output rec, Francesco requires a high quality. So a tube stage is mandatory.
- Could a 6DJ8 SRPP (with a capacitor output) work properly?
- Is the connection reported in the schematic correct?
We are a little worried about the last connections (for phone e rec) in the output stages … We think that they could modify the load of output stages.
Ciao
Antonio e Francesco
I have discussed with Francesco about mixer requirements and it seems that it remains to add some sections …before organize the layout of the PCBs. In particular, it is necessary to include
1) An output (driving 30 mt of cables) for monitors
2) A mic amplifier for the monitor mic (through which the technician of the mixer can talk to musicians).
3) A headphone output
4) Vu meters for the 4 output stages
5) Output recording.
As to point 1, 2, and 3 we think that high audio quality is not necessary in these sections, so a single chip solution could be the best.
1) As to the first one (monitor outputs), we have thought to THAT IC, a nice single chip solution for the balanced line driver THAT Corporation 1606/1646 OutSmarts Balanced Line Driver ICs
- What do you think about it ? Have you other solutions?
- Could we directly connect the 4 outputs of TL082 to 4 THAT 1646 ICs (without capacitors) ?
2) As to monitor mic we think that a single chip solution for both should be the best.
- Do you suggest a particular chip?
- How to connect the output of chip with the four monitor channels ? Is a 22K R enough for a distribution of the signal ? … or we have to put a R to ground for each channel too?
\\\\\\\\\\\\\\\\\|--22K -> monitor 1 (to R35)
Chip---->------ |--22K -> monitor 2 (to R36)
\\\\\\\\\\\\\\\\\|--22K -> monitor 3 (to R37)
\\\\\\\\\\\\\\\\\|--22K -> monitor 4 (to R38)
3) As to headphone output we think that a single chip solution for both should be the best.
- Have you some suggestion for it ?
- Are correct the connection reported in the schematic?
4) For the master fader we think that a great display Vu meters are necessary.
- Which Vu meters do you suggest? Led type or analog type?
5) As to output rec, Francesco requires a high quality. So a tube stage is mandatory.
- Could a 6DJ8 SRPP (with a capacitor output) work properly?
- Is the connection reported in the schematic correct?
We are a little worried about the last connections (for phone e rec) in the output stages … We think that they could modify the load of output stages.
Ciao
Antonio e Francesco
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I am sorry I cannot help you with this question. I am not familiar with this device but looking at its data sheet I think it should be able to drive the 30mt cable OK.
Again, sorry I cannot help with this question - I have no experience of chips of this type. I do not understand your diagram showing how the mic is to be connected. VCan you explain please?
Sorry, once again this is not my area of expertise.
I think this is a personal preference. I like analogue meters but that is just my preference because it reminds me of the good old days.
A 6DJ8 SRPP should be OK for this.
At present you have a very simple fixed sub-mix fro the phones and rec outputs. It might be better to have separate faders for bass, vocal and instrument so you can balance their relative levels for recording. It would then be normal practice for the headphones to be switchable to each individual output or the the stereo rec output.
Cheers
Ian
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