Bonsai's practical buffer
Idea for this buffer came from forum member who needed simple volume control for the input of Bonsai's nx power amplifier. PMI volunteered to design small pcb for a possible future GB.
Design criteria for this buffer are:
1. pcb will have onboard pot that will allow simple front plate one hole mounting. pcb will be small enough to fit any power amp chassis
2. Pot will be high quality ALPS Blue Velvet RK27. Pot shaft will be used to hold the pcb to front plate
3. pcb will have simple zener regulator + RC filtering onboard so that the whole circuit could use existing power amp supply/transformer. Additional holes could be provided on pcb for those who intend to use better offboard regulation
With the addition of rear toggle switch placed between two pairs of RCA connectors and using short unshielded wires, this buffer pcb could transform any power amp into integrated amp with two line inputs. With the addition of Lorlin 3way/4pole rotary switch and using shielded cables (with or without extension shaft) this buffer could transform any power amp into integrated amp with 3 line inputs.
If current consumption of the circuit is known, we could calculate voltage dropping resistor for any power amp supply...
Thanks for starting this Ivanlukic. I propose to cascode the input JFET to improve PSRR. It will have to wait though until I am back from vac and can play with it in LTspice.
Mechanically speaking, this could also be mounted using a U bracket with lips at the rear of the amplifier to avoid running screened cable connections to the front panel and the possibility of noise. In this case, you would need to use a shaft extension coupler to couple the knob to the rear of the amplifier.
Load for this buffer will be 2k Ohms min, 10k nominal which should cover just about any eventuality.
Anyway, I'll do some work on it towards the end of next week.
Will this be a bf862 (SMD sot23) as jFET?
1. please decouple Zener diodes with 47...100 µF 63 V caps and use separate 100 Ohm resistors in the supply lines for each channel. Each channel will have its own 2 x 100 µF decoupling caps on the supply lines. You could also add pads so LM317/337 can be used. Or better, a nice dual (SMD !!!!) reg like the LT3032-12:
You will need to lower the input voltage to under 20 V so either preregs or 2 x 18 V 1 W Zeners should be used.
Unobtainable here but still:
I still use LM325, XR4195 and M5230 but those are very old and possibly hard to find. Especially the M5230 is a nice low noise chip.
2. Some RC input filtering would not hurt either. - 3dB point around 100 kHz.
3. An output GND reference resistor in each channel would make the PCB more versatile.
4. Additional pads for an optional input cap for those that have DC coupled sources with offset would be nice too.
I am not going to design pcb, it will be done by PMI. And the circuit design is Bonsai's, and not yet finished. I am just facilitator and promoter of this idea, hoping to help. We shall have to achieve some measure between complexity of the circuit, PSU and size of pcb. I think that your comments are more than welcome and does not require too much space on the pcb. This is collective effort, we want as much relevant comments as possible.
I am proposing
1. CCS for the JFET will be the two transistor type (flexible and easy)
2. Bootstrap the JFET for input capacitance mitigation - use BIP device to buff output and drive bootstrap cap and follower
3. Agree to add input filter - but then amplifier input filter must be modded or removed.
4. Agree that each channel should be separately filtered and decoupled
5. 3 term regs will add complexity . . . Do we really need that?
6. CCS and bootstrap will make PSRR VERY high plus decoupling on top of that
7. Small PCB with all components . . .
8. Important that the buffer can use any JFET with Idss above about 10 to 12 mA and not SMD. I am thinking J113 type devices - any recommendations gladly accepted
9. All trannies will be BC547/557 C types
10. Input DC blocking cap option - ok will add this. The volume pot will therefore be spec'd for 10k log or higher.
I´m not sure if a high PSRR is something required and that the added complexity will improve matters, especially sonic behaviour.
Cascoding the JFET with a second JFET -eg the very fine 4393/4391 combination- will only benefit with high load impedances >20k and high Ids.
With loading below 20k the single JFET could be better THD-wise.
Maximum voltage swing will be reduced by the cascode to ~4-5Vrms
Cascoding reduces power heat losses with the JFETs.
If the CCS is tuned to the Idss of the JFET one might even omit with the DC-blocking cap. I´d suggest to use a second identical JFET as CCS in that case though, because of thermal stability.
It's the choice between either a high PSRR or a very good power supply (I would do both). A power supply is a very essential part for optimal peformance.
The original idea was to keep it as simple and practical as possible even if the lab performance will be less than ideal. I think that we should keep the basic concept of 1 j-fet + 2 bjt from the post #1 and refine PSU a little bit with separate filtration for each channel suggested by jean-paul. Also, input low pass filter will make it more versatile, plus input coupling cap, but output resisitor to ground is always provided by the input of power amp, although a place on the pcb for such resisitor will also make the whole pcb more versatile and does not require much space.
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