http://myweb.tiscali.co.uk/nuukspot/decdun/gainclonepre2.html
AIA try a search for a one jfet transistor buffer or preamp.
There is the +/-15 supply in the amp, a two transistor preamp maybe in order.
I have two amp that had a small hum problem, bypassing the preamp section solve the problem in both amps.
AIA try a search for a one jfet transistor buffer or preamp.
There is the +/-15 supply in the amp, a two transistor preamp maybe in order.
I have two amp that had a small hum problem, bypassing the preamp section solve the problem in both amps.
swapping ICs
Have anybody tried,add sockets for the ic(s). The IÇs that are installed are junk. I think a decent op amp will improve things.
Have anybody tried,add sockets for the ic(s). The IÇs that are installed are junk. I think a decent op amp will improve things.
I put sockets on the board and the crappy 4558s with two tl72s. There is some improvement in the midrange less harsh. Next step, bypass all the 4.7uf electrolytic with some higher quality film capacitors.
After that, I will replace the volume pot with a $.50 pot I purchase. For $.50 the pot turn out to be very very good, Hell the pot is good period!
Btw there is some gain on the preamp board.
After that, I will replace the volume pot with a $.50 pot I purchase. For $.50 the pot turn out to be very very good, Hell the pot is good period!
Btw there is some gain on the preamp board.
Accurian
In the mod posted for direct input from the line out jacks, there is no need to switch the inner plug to the outer position after removing the feed from the RF board. A quick look on the back of the preamp board shows them connected already. Also, I removed the power feed (plug by top boards transformer)to the RF board and put it back in place. This makes the power transformer on that board no longer a potential source of hum. The amp can be returned to it's normal state by plugging the connectors back in! Bob Eiser
In the mod posted for direct input from the line out jacks, there is no need to switch the inner plug to the outer position after removing the feed from the RF board. A quick look on the back of the preamp board shows them connected already. Also, I removed the power feed (plug by top boards transformer)to the RF board and put it back in place. This makes the power transformer on that board no longer a potential source of hum. The amp can be returned to it's normal state by plugging the connectors back in! Bob Eiser
I completely removed the RF board. I don't plan on returning it to it's original state since I have 3 more units 🙂
-Ken
OK, I finally have had time to tear into this thing. I removed the RF board and switched the input (though it apparently does not need to be done). I removed the entire preamp board and now I need a little guidence.
Can I just wire in a volume pot on its own ignoring the rest of the board, or do I need the 4.7 uF caps?
What about the +15v, -15v and LED leads on the wire harness headed to the amp board. Do they need to be reconnected?
Thanks,
AIA
Can I just wire in a volume pot on its own ignoring the rest of the board, or do I need the 4.7 uF caps?
What about the +15v, -15v and LED leads on the wire harness headed to the amp board. Do they need to be reconnected?
Thanks,
AIA
Yes to both. If you leave out the cap,check the dc at output. I tried it,I got 4.5mv at output. If your source has low dc offset,you should be fine.AIA said:
AIA said:
No,with the preamp board gone, the +/-15v is not need.
Does that mean that the 4.7uF cap is meant to balance the dc offset at the output?
Should I expect a large dc offset if the source is a pc soundcard generating mp3s?
Thanks,
AIA
Should I expect a large dc offset if the source is a pc soundcard generating mp3s?
Thanks,
AIA
DC
The capacitors block the DC from getting to the amps input. For the lowest bass response, no cap is desirable. If the amp responds to DC, it will amplify the DC by it's gain. Check the DC output from your soundcard with a voltmeter on the 200 mv range. Bob
The capacitors block the DC from getting to the amps input. For the lowest bass response, no cap is desirable. If the amp responds to DC, it will amplify the DC by it's gain. Check the DC output from your soundcard with a voltmeter on the 200 mv range. Bob
I stumbled on this thread last week and was lucky enough to find last 2 of these at local Rat Shack store. I did the input mod on one and it sounds ok but not good enough to have much use for me. I actually bought these for the transformers. I want to use them for LM3886 mono block. The voltage of the transformers looks good but is 2 amps enough?
As someone who used to work for Radio shack some years ago: On discontinued merchandise, they have, (or used to have) a listing of local stores which have it.
It depends on how friendly you are with the salesperson if he wants to go through the effort of either telling you who still has it, or to arrange to have it shipped to his store. So it you have a friendly relationship with someone at your local Radio Shack, he would be the one to ask.
This applied to how Radio Shack was a few years ago, assuming things have not changed.
It depends on how friendly you are with the salesperson if he wants to go through the effort of either telling you who still has it, or to arrange to have it shipped to his store. So it you have a friendly relationship with someone at your local Radio Shack, he would be the one to ask.
This applied to how Radio Shack was a few years ago, assuming things have not changed.
Thank you for the advice. I guess I was nice enough or lucky to get the 2 amps. My question is will the 28v 2amp transformers work well in a LM3886 gainclone? Thanks again.
Bruce
Bruce
This post hasn't had an addition in 9 months!
I just purchases the last Accurian Receiver from the local Radio
Shack (Santa Rosa, CA) for $16. I pulled the "wireless" board
and rerouted the AC wires while adding a good twist. I also
twisted the RCA input wires. Measured the DC offset and it was
under 5mV for each channel. /2W
I plan to use it as a preamplifer. I will add a small board inside and
attach it to the post that held the "wireless" board. There will be
two L-pads and these will be wired to a two new sets of RCAs.
I tested the unit with a 15K ohms / 1.1K ohm external L-pad that
drops the voltage 24dB with a 20k ohm load. . . everything worked great.
So, I will make the new internal L-pad with a 220 ohm /2W and
a 47 ohm to ground resistor. With my Rotel RB-850 as the load
it will result in -15 dB of the Accurain Voltage
The Accurian should develop 5.69V across the L-pad which will pass
95.3mA of current and produce 1V at the 27k ohm amplifier input
producing 45W/8ohm.
If the Accurain develops 14.14V (equal to 25W/8ohms), then
there will be 2.48V at the amplifer input. . . that is 7.9dB of gain.
I think the Accurain can develop closer to 23V with the higher
resistance at the output. . .so, it can easily have a gain of close
to 12dB.
I just purchases the last Accurian Receiver from the local Radio
Shack (Santa Rosa, CA) for $16. I pulled the "wireless" board
and rerouted the AC wires while adding a good twist. I also
twisted the RCA input wires. Measured the DC offset and it was
under 5mV for each channel. /2W
I plan to use it as a preamplifer. I will add a small board inside and
attach it to the post that held the "wireless" board. There will be
two L-pads and these will be wired to a two new sets of RCAs.
I tested the unit with a 15K ohms / 1.1K ohm external L-pad that
drops the voltage 24dB with a 20k ohm load. . . everything worked great.
So, I will make the new internal L-pad with a 220 ohm /2W and
a 47 ohm to ground resistor. With my Rotel RB-850 as the load
it will result in -15 dB of the Accurain Voltage
The Accurian should develop 5.69V across the L-pad which will pass
95.3mA of current and produce 1V at the 27k ohm amplifier input
producing 45W/8ohm.
If the Accurain develops 14.14V (equal to 25W/8ohms), then
there will be 2.48V at the amplifer input. . . that is 7.9dB of gain.
I think the Accurain can develop closer to 23V with the higher
resistance at the output. . .so, it can easily have a gain of close
to 12dB.
There is talk of keeping the 4.7uF cap or replacing it with better
caps. . .couldn't you replace the cap with an input transformer?
caps. . .couldn't you replace the cap with an input transformer?
quick question
The answers to my questions are probably obvious, but I am somewhat new so thanks to anyone who can help me out. Do the 15+ and 15- (wiring harness connections) on the amp board go to or from the tone control board? Or to reword it, does the tone control board get its power from the amp board? Is the 15 volts regulated or unregulated and does anyone know how much mAs it has on tap?
The answers to my questions are probably obvious, but I am somewhat new so thanks to anyone who can help me out. Do the 15+ and 15- (wiring harness connections) on the amp board go to or from the tone control board? Or to reword it, does the tone control board get its power from the amp board? Is the 15 volts regulated or unregulated and does anyone know how much mAs it has on tap?
The +/- 15VDC supply is for the preamp board (if you take a close
look there are two op-amps on that board; there are some zeners
in the power supply and since it is running op-amps, then I
would be quite certain that it is regulated. . .but not too much current.
Notice also that most of the resistors on the preamp board are
1/8W carbons. . .again, not too much current; there is a spot on the
main board for a 3-pin voltage regulator and heat sink. . .but unused,
there would be two for a split supply. . .
But it doesn't need it that much current. . .it is a preamp. Someday
soon I need to pull the board an make a schematic. . . that would
tell for absolute certain.
look there are two op-amps on that board; there are some zeners
in the power supply and since it is running op-amps, then I
would be quite certain that it is regulated. . .but not too much current.
Notice also that most of the resistors on the preamp board are
1/8W carbons. . .again, not too much current; there is a spot on the
main board for a 3-pin voltage regulator and heat sink. . .but unused,
there would be two for a split supply. . .
But it doesn't need it that much current. . .it is a preamp. Someday
soon I need to pull the board an make a schematic. . . that would
tell for absolute certain.
There is a zobel filter on the output. . . values to come. . . not sure if
it is true. . .but increasing the cap value will lower the freq response?
it is true. . .but increasing the cap value will lower the freq response?
Thanks for all of your help so far. Have you had a chance to see if it is in fact a zobel filter?
Pulled the board and traced the circuit. . . seems to always be parallel
to the output to ground irrespective of the output relays. . .then
the load is parallel to it. . .I stink at reading values on resistors and
caps. . .seems like a small value for zobel.
2W output (Red,Red,Silver,Gold) 0.22 ohms
1W zobel (Yellow,Violet,Gold,Gold) 4.7 ohms
to the output to ground irrespective of the output relays. . .then
the load is parallel to it. . .I stink at reading values on resistors and
caps. . .seems like a small value for zobel.
2W output (Red,Red,Silver,Gold) 0.22 ohms
1W zobel (Yellow,Violet,Gold,Gold) 4.7 ohms
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