B1 Buffer Preamp

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lae2

Member
Joined 2010
Jen-B said:
Yes! It's not in the audio line, and when the unit is powered up it is reverse biased anyway. Only when you turn the unit off will the diode be forward biased and then it will drain C2 via R4 and LED D2. So you will be fine.
Click to expand...

From an academic standpoint, isn't R3 a lower resistance path and thus obviates D1? If someone omitted D2, R3 still provides at bleed path. Is it possible for D1 to mediate bleeding through the FETs?
 
J

jleaman

Banned
Joined 2002
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A

Atilla

Member
Joined 2008
I'm doing my casework now, mostly p2p, although the PSU and the transistors themselves are on protoboards. I was wondering why every input has a 1M resistor to ground, instead of using only 1 resistor after the switch. Couldn't I do that instead? I've got 4 inputs in my case, and the switch switches both the signal and ground for all of them.

The only difference in that config will be that any sources that are on will be open-circuit rather than loaded with 1 megaohm. I can't see a problem with that, can you?
 
O

orpheus

Member
Joined 2004
Reversed voltage at input

I quite stupidly attached a battery power supply to my B1 with the +on the ground and the - on the pin (i.e. backwards). I now get a distorted sound through the B1. I am thinking that I blew the 2SK370's, and that they need to be replaced. Is this the logical repair step?

I was also running an optocoupler as the volume control, and it seems to be OK when I measure the resistance of the optocouplers. It is possible/probable that I blew the optocouplers as well?

I tried the B1 by itself without the volume control (connecting W and CW, input and output for the pot), and the sound had the same distortion.

I can order 2SK170's and match them. Are the 370's better or different? I did some searches, but I couldn't find a comparison. I might ask Pass Labs if they will sell me a replacement set since I did buy my board and jfets from them originally.

Thank you for your help and suggestions.

Aaron.
 
U

udailey

Member
Joined 2008
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Orpheus,
If you measure resistance on your optocouplers then they are working. Especially if they move around when you change the power to them. If you hook them up backward to the power supply you CAN blow them, but it has only happened once to me and I have done that several times. If you want to rule them out then just hook them up as a pot and remove the B1 and see if you have the distortion.
I hope you are regulating or otherwise protecting them. If you use the B1's 18V to the optocouplers, and if you are using LDRs which I assume you are, then 18V will destroy them very fast if not properly protected. I will assume you are regulating to appropriate voltage since they are not destroyed.
If you need some 2sk170's that are matched let me know and I will send you a pair or two.
Uriah
 
zener_diode

zener_diode

Member
Joined 2007
Atilla said:
I'm doing my casework now, mostly p2p, although the PSU and the transistors themselves are on protoboards. I was wondering why every input has a 1M resistor to ground, instead of using only 1 resistor after the switch. Couldn't I do that instead? I've got 4 inputs in my case, and the switch switches both the signal and ground for all of them.

The only difference in that config will be that any sources that are on will be open-circuit rather than loaded with 1 megaohm. I can't see a problem with that, can you?
Click to expand...

The 1M resistor is there to bleed down any DC voltage that might have
developed when the input was not selected. There is explanation of it in the
Pass B1 article.
 
O

orpheus

Member
Joined 2004
udailey said:
Orpheus,
If you measure resistance on your optocouplers then they are working. Especially if they move around when you change the power to them. If you hook them up backward to the power supply you CAN blow them, but it has only happened once to me and I have done that several times. If you want to rule them out then just hook them up as a pot and remove the B1 and see if you have the distortion.
I hope you are regulating or otherwise protecting them. If you use the B1's 18V to the optocouplers, and if you are using LDRs which I assume you are, then 18V will destroy them very fast if not properly protected. I will assume you are regulating to appropriate voltage since they are not destroyed.
If you need some 2sk170's that are matched let me know and I will send you a pair or two.
Uriah
Click to expand...

Thank you for responding to my post. I'm using the LDR's that I bought from you with the circuit board from dvb-projekt, which has a regulated supply installed. I am powering the optocouplers with a rechargeable 9 volt battery for now. The reason that I thought the optocouplers might have blown is that the ground on the B1 is connected to the ground on the optocoupler board, so there was voltage on the ground on the board. I'll have to rig up some cables to use it as a standalone volume control. Perhaps that will make a good stand in while my B1 is down.

Chrismercurio:

I was using a 12 volt sealed lead acid battery, about 3 amps of current. It is under spec for the B1, but it was working (that is until I plugged one in backwards).



One question, if I am using a LM7805 regulated 5 volt supply for the optocouplers, could I power it from the LED power supply on the B1 at +18 volts? I'm not planning on using an LED for the B1, and that would make for an easy connection. Might tax the regulator too much though, what do you think?

Thanks again for your response and assistance.

Best,
Aaron.
 
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