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Well, here I am, following in regis' footsteps...
I am currently building an attenuator for a friends Hotrod deluxe valve amp, weighing in at 40w.
having looked around, I decided that it would be way too powerful for a single variable L-Pad to suffice, so it will need a slightly more detailed approach.
So far I have the following parts headed my way:
2 x 100w 4ohm wirewound resistors (alu shell)
2 x 50w 8ohm wirewound resistors (alu shell)
6.8uF 400v MKP Audiophilar cap
12uF 400v MKP Audiophilar cap
2 x 6.5mm PCB mount jack socket
2' 6.5mm patch cable
PCB 90mm x 150mm
on-off-on DPDT switch (15a @ 250V)
2 x on-on DPDTswitch (15a @ 250v)
Computer PSU to mount it in (using the 80mm fan for cooling)
12v 1A DC power supply (with AC power input)
Optional: additional 120mm or 92mm cooling fan.
Other pertinent info:
The amp is by default an 8 ohm output, but has a second output jack into which an 8 ohm speaker/cab can be plugged. This causes the amp to switch to an 4 ohm output. (thanks regis for that info). Regis suggested I could use a stereo 100w 8 ohm L-Pad in dual ganged mode to make it effectively a 200w mono 4 ohm L-pad. The amp could have a dead end jack plugged into the second port, or have the 4/8 switch hard wired in.
This would create a second wire to hardwire in, and also mean more engineering for both the ease of disconnecting/removing the attenuator, and also impedance matching at the speaker end (being an 8 ohm speaker)
My plan:
Use a standard AC cable hardwired into the power input on the amp. This will be plugged into the DC power supply, and in turn into the attenuator. Means that whenever the amp is switched ON it will also power the fan on the attenuator, regardless of attenuator configuration.
Attenuator plugged in as you would expect:
AMP > patch lead > Attenuator > Speaker (which has its own 6.5mm jack)
The attenuator will be 8 ohm only.
Having decided that the power throughput would be too much for a single L-Pad to handle, I thought it would be a good idea to put resistors in front of the L-pad, IE a fixed -3db 8ohm load in L series with the L-pad. - basically the same design as regis' Ampoline attenuator, with an L-Pad instead of resistors as the second stage. I then decided it would probably be easy also to make the first stage a -3db, and the second an identical -3db, but with the second stage having an L-pad which theoretically means it can go to -infinity.
Will do up a proper diagram and attach it.
Comments/thoughts/suggestions?
I am currently building an attenuator for a friends Hotrod deluxe valve amp, weighing in at 40w.
having looked around, I decided that it would be way too powerful for a single variable L-Pad to suffice, so it will need a slightly more detailed approach.
So far I have the following parts headed my way:
2 x 100w 4ohm wirewound resistors (alu shell)
2 x 50w 8ohm wirewound resistors (alu shell)
6.8uF 400v MKP Audiophilar cap
12uF 400v MKP Audiophilar cap
2 x 6.5mm PCB mount jack socket
2' 6.5mm patch cable
PCB 90mm x 150mm
on-off-on DPDT switch (15a @ 250V)
2 x on-on DPDTswitch (15a @ 250v)
Computer PSU to mount it in (using the 80mm fan for cooling)
12v 1A DC power supply (with AC power input)
Optional: additional 120mm or 92mm cooling fan.
Other pertinent info:
The amp is by default an 8 ohm output, but has a second output jack into which an 8 ohm speaker/cab can be plugged. This causes the amp to switch to an 4 ohm output. (thanks regis for that info). Regis suggested I could use a stereo 100w 8 ohm L-Pad in dual ganged mode to make it effectively a 200w mono 4 ohm L-pad. The amp could have a dead end jack plugged into the second port, or have the 4/8 switch hard wired in.
This would create a second wire to hardwire in, and also mean more engineering for both the ease of disconnecting/removing the attenuator, and also impedance matching at the speaker end (being an 8 ohm speaker)
My plan:
Use a standard AC cable hardwired into the power input on the amp. This will be plugged into the DC power supply, and in turn into the attenuator. Means that whenever the amp is switched ON it will also power the fan on the attenuator, regardless of attenuator configuration.
Attenuator plugged in as you would expect:
AMP > patch lead > Attenuator > Speaker (which has its own 6.5mm jack)
The attenuator will be 8 ohm only.
Having decided that the power throughput would be too much for a single L-Pad to handle, I thought it would be a good idea to put resistors in front of the L-pad, IE a fixed -3db 8ohm load in L series with the L-pad. - basically the same design as regis' Ampoline attenuator, with an L-Pad instead of resistors as the second stage. I then decided it would probably be easy also to make the first stage a -3db, and the second an identical -3db, but with the second stage having an L-pad which theoretically means it can go to -infinity.
Will do up a proper diagram and attach it.
Comments/thoughts/suggestions?
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