eguth
pfm Member
POST #6 (see below) ENDS WITH ALL THE PHOTOS AND DIAGRAMS THAT WERE SOMEHOW DELETED FROM THIS FIRST POST.
In my Audio Conversions article (ISSUE 12, p.14-16) I gave complete details of the passive preamp I built to control my (then) bi- amped system.
Since then I discovered the benefits and pleasures of tri-amping. And so I designed a new switched attenuator to bolt onto the old passive preamp case. The outside of the preamp now sports a bank of toggle switches: neat, elegant and a joy to use.
The principle of my new design is: "the best switch is NO switch; the second best switch is a toggle switch".
The circuit is not my own. I chose Graham Nalty's 'ground switching' circuit for which I give him full credit. [see his excellent Audio Conversions article "The Ultimate in Volume Controls", AC 3rd ed., Sept. 1988, p.26-30].
His basic idea is a single switch in the signal path. Nalty uses a fixed resistor shunted to ground by resistors of different values
The new twist is mine: to use ONLY toggle switches. A toggle is a joint consisting of two arms almost in line with each other. The arms are connected with an elbow pivot. This type of switch has a positive snap action. I have found that it sounds very good, indeed, and is foolproof. Ground switching is completely silent. When I throw a toggle all I hear is the mechanical soft click of the toggle: most satisfying.
Fig. 2 WIRING DIAGRAM
I almost always leave the preamp (volume) toggle switched to position 14, the normal volume setting. Occasionally I switch to position 11 and occasionally to position 18. 'Occasionally' means 'not even once a month'. Other than that- perhaps once a year- I find a recording that needs much lower volume- between positions 4 & 8. And once every year I go for position 21. That is LOUD! It is the only position where there is only one resistor in circuit. This I do on bonfire night when I listen to the 1812 Overture (preceded by Handel's Water Music, I might add)- from the safety of the next room with the door firmly closed. They can hear the cannon blasts from my horn subwoofer across the road and down a few houses. That is not very far away, only because my windows and doors are kept shut. The remaining toggle positions? I use them only, perhaps, once every 80 years and then only to test that they are still working.
When changing volume I always throw the toggle I want to go to first, leaving the existing toggle switched in until the new toggle has been thrown. Then I put the previous toggle to off. This prevents the default position (position 21) from sounding during the change in volume. It sounds complicated but its not and it soon becomes automatic.
NOTES
NORMAL POSITION
Best method for determining resistor values:
1) Take reading of your normal potentiometer setting for the most frequently used volume setting.
2) Use 10K for RO.
3) Try 4.7K for R21 (or lower value by 10dB). You will need to take the proportion of your normal pot reading from input to wiper of 10K in order to use 10K and to determine how much of an increase or decrease 10K is on your normal reading. This percentage, and a similar one with respect to reading of wiper to ground will determine your R14 value equivalent volume with R21 in parallel and with input resistor (with RO=10K) as your normal pot setting. Take readings of your highest and lowest pot settings also.
4) Calculate the resistor needed in parallel with R21 to make R14 the equivalent in resistance of your normal pot setting.
5) Choose highest value to try for R20, lowest value to try for R1. I suggest that you use 1W metal films for R1-R20- 1W sound better. Insulate the ground wire between resistor connexions with pieces of PTFE tubing. Bend upper and lower pins of switches up and down to aid soldering. Bend middle (off) positions of switches outwards for the same reason.
THE NORMAL VOLUME POSITION: SW4 toggle thrown DOWN, R14 IN PARALLEL with R21. The values are given using a cartridge of 0.4mV output and my buffer stage. My buffer stage also provides 10dB gain, so with this passive preamp and NO BUFFER STAGE WITH GAIN and a 0.4mV output cartridge you need to increase the values of R1-R20 by approx 10dB, or use same values and INCREASE R21. It is best to use standard values for R1-R20. The values I use are all standard values except for R14 which is a composite of 3 resistors in parallel. In my version, positions 1-4 cannot be used because they attenuate too much and cause hum. You could alter these values to bring R1-R10 closer steps, and start R1 at higher resistance- at least 390R.
N.B. To INCREASE gain of the ENTIRE switched attenuator (GENERALLY) INCREASE the value of R21. To DECREASE the gain (generally) of the entire attenuator DECREASE the value of R21. IN EITHER case the values of R1-R20 remain unaltered. To DECREASE gain (GENERALLY) INCREASE the value of R0 (R0 is easier to desolder and resolder than R21).
[/COLOR
Fig. 3
NORMAL POSITION (1 channel )
R14 IS SHOWN SWITCHED IN PARALLEL WITH R21 VIA SW4
After a good deal of experiment with different resistors I chose a mixture. It is useful to have the means of changing the quality of the sound. This can be used to mask poor recordings, badly scratched ones, or other nasties. I can throw 2 or sometimes even 3 toggles together to put a sound smokescreen over the sound: you cannot do that with a rotary switch. Those 'Doubting Thomases' who believe that 'all resistors sound the same' are in for a rude awakening provided that their system is good enough and that they do not have cloth ears.
I ended up using Vishay bulk foils for R21, Caddock TFO 20s for R0. At positions 9,10,16,19 Holco H2, and at 11,14 Vishay bulk foil. At 17 & 20 I use Caddock TFO20. All other resistors are ordinary metal films: Beyshlag. At this point I mention that I am a great fan of Holcos but I understand that Holsworthy, the manufacturers, have now changed the design and that the new version does not sound as good. Supplies of the old version are getting scarce but you may be able to obtain these on the net. I do not like the sound of 15ppm Welwyns. I always use 1W rating where posssible. I can't use the best resistors (Vishay bulk foil) throughout; they are just too revealing of defects in my system.
I used only gold- plated, sealed toggle switches designed for low level signal switching applications: these are not expensive. You will need, I predict, not more than ten of them. I used miniature ones obtained from RS for the volume bank, and large ones for source switching. You can pull the signal plugs in and out instead of source switching, but I feel that switches are a compromise worth compromising over. There are only 2 sources in my version, one source switch per channel. I also have one large toggle to alter the vol. of the subwoofer; but I could have used a fixed resistor instead because I never alter the toggle position. Roederstein 1W are fitted to this toggle.
I have used the shortest possible runs of interconnect (I use Belden 78ohm data cable- BICC electronic cables H8065- reviewed by Martin Colloms years back as being almost as good as the best audiophile cable he then tested). It is inexpensive, and I bought a whole roll and have never yet either looked back or come to the end of it. My interconnect runs are much less than 1 metre from signal to passive and less than a metre from passive to amps. I say I am tri-amped but I actually use 4 amps; 2 monoblocks, 1 stereo amp and 1 stereo amp bridged to mono.
Everything within the passive is hard- wired using silver solder and, mostly, 99.99% pure silver wire -where necessary sleeved in Teflon tubing. I made certain that all resistor leads, switch terminals and wires were squeaky clean before soldering.
The total cost of this passive preamp is low; compare the price of miniature gold- plated toggle switches with gold - plated rotary ones.
What next? I'm thinking of quad-amping. I needed a buffer stage to cope with the added interconnect that tri-amping brought, but, again, this was a compromise worth compromising for. I am also thinking of moving speakers- no not shifting them to a new location- but speaker ENCLOSURES that MOVE (while the music plays).
Eguth[/SIZE[/[/COLOR][/SIZE]COLOR[/COLOR]]][/FONT][/SIZE][/FONT][/COLOR][/SIZE]
A TOGGLE SWITCHED ATTENUATOR
In my Audio Conversions article (ISSUE 12, p.14-16) I gave complete details of the passive preamp I built to control my (then) bi- amped system.
Since then I discovered the benefits and pleasures of tri-amping. And so I designed a new switched attenuator to bolt onto the old passive preamp case. The outside of the preamp now sports a bank of toggle switches: neat, elegant and a joy to use.
The principle of my new design is: "the best switch is NO switch; the second best switch is a toggle switch".
The circuit is not my own. I chose Graham Nalty's 'ground switching' circuit for which I give him full credit. [see his excellent Audio Conversions article "The Ultimate in Volume Controls", AC 3rd ed., Sept. 1988, p.26-30].
His basic idea is a single switch in the signal path. Nalty uses a fixed resistor shunted to ground by resistors of different values
The new twist is mine: to use ONLY toggle switches. A toggle is a joint consisting of two arms almost in line with each other. The arms are connected with an elbow pivot. This type of switch has a positive snap action. I have found that it sounds very good, indeed, and is foolproof. Ground switching is completely silent. When I throw a toggle all I hear is the mechanical soft click of the toggle: most satisfying.
Fig. 2 WIRING DIAGRAM
I almost always leave the preamp (volume) toggle switched to position 14, the normal volume setting. Occasionally I switch to position 11 and occasionally to position 18. 'Occasionally' means 'not even once a month'. Other than that- perhaps once a year- I find a recording that needs much lower volume- between positions 4 & 8. And once every year I go for position 21. That is LOUD! It is the only position where there is only one resistor in circuit. This I do on bonfire night when I listen to the 1812 Overture (preceded by Handel's Water Music, I might add)- from the safety of the next room with the door firmly closed. They can hear the cannon blasts from my horn subwoofer across the road and down a few houses. That is not very far away, only because my windows and doors are kept shut. The remaining toggle positions? I use them only, perhaps, once every 80 years and then only to test that they are still working.
When changing volume I always throw the toggle I want to go to first, leaving the existing toggle switched in until the new toggle has been thrown. Then I put the previous toggle to off. This prevents the default position (position 21) from sounding during the change in volume. It sounds complicated but its not and it soon becomes automatic.
NOTES
NORMAL POSITION
Best method for determining resistor values:
1) Take reading of your normal potentiometer setting for the most frequently used volume setting.
2) Use 10K for RO.
3) Try 4.7K for R21 (or lower value by 10dB). You will need to take the proportion of your normal pot reading from input to wiper of 10K in order to use 10K and to determine how much of an increase or decrease 10K is on your normal reading. This percentage, and a similar one with respect to reading of wiper to ground will determine your R14 value equivalent volume with R21 in parallel and with input resistor (with RO=10K) as your normal pot setting. Take readings of your highest and lowest pot settings also.
4) Calculate the resistor needed in parallel with R21 to make R14 the equivalent in resistance of your normal pot setting.
5) Choose highest value to try for R20, lowest value to try for R1. I suggest that you use 1W metal films for R1-R20- 1W sound better. Insulate the ground wire between resistor connexions with pieces of PTFE tubing. Bend upper and lower pins of switches up and down to aid soldering. Bend middle (off) positions of switches outwards for the same reason.
THE NORMAL VOLUME POSITION: SW4 toggle thrown DOWN, R14 IN PARALLEL with R21. The values are given using a cartridge of 0.4mV output and my buffer stage. My buffer stage also provides 10dB gain, so with this passive preamp and NO BUFFER STAGE WITH GAIN and a 0.4mV output cartridge you need to increase the values of R1-R20 by approx 10dB, or use same values and INCREASE R21. It is best to use standard values for R1-R20. The values I use are all standard values except for R14 which is a composite of 3 resistors in parallel. In my version, positions 1-4 cannot be used because they attenuate too much and cause hum. You could alter these values to bring R1-R10 closer steps, and start R1 at higher resistance- at least 390R.
N.B. To INCREASE gain of the ENTIRE switched attenuator (GENERALLY) INCREASE the value of R21. To DECREASE the gain (generally) of the entire attenuator DECREASE the value of R21. IN EITHER case the values of R1-R20 remain unaltered. To DECREASE gain (GENERALLY) INCREASE the value of R0 (R0 is easier to desolder and resolder than R21).
[/COLOR
Fig. 3
NORMAL POSITION (1 channel )
R14 IS SHOWN SWITCHED IN PARALLEL WITH R21 VIA SW4
After a good deal of experiment with different resistors I chose a mixture. It is useful to have the means of changing the quality of the sound. This can be used to mask poor recordings, badly scratched ones, or other nasties. I can throw 2 or sometimes even 3 toggles together to put a sound smokescreen over the sound: you cannot do that with a rotary switch. Those 'Doubting Thomases' who believe that 'all resistors sound the same' are in for a rude awakening provided that their system is good enough and that they do not have cloth ears.
I ended up using Vishay bulk foils for R21, Caddock TFO 20s for R0. At positions 9,10,16,19 Holco H2, and at 11,14 Vishay bulk foil. At 17 & 20 I use Caddock TFO20. All other resistors are ordinary metal films: Beyshlag. At this point I mention that I am a great fan of Holcos but I understand that Holsworthy, the manufacturers, have now changed the design and that the new version does not sound as good. Supplies of the old version are getting scarce but you may be able to obtain these on the net. I do not like the sound of 15ppm Welwyns. I always use 1W rating where posssible. I can't use the best resistors (Vishay bulk foil) throughout; they are just too revealing of defects in my system.
I used only gold- plated, sealed toggle switches designed for low level signal switching applications: these are not expensive. You will need, I predict, not more than ten of them. I used miniature ones obtained from RS for the volume bank, and large ones for source switching. You can pull the signal plugs in and out instead of source switching, but I feel that switches are a compromise worth compromising over. There are only 2 sources in my version, one source switch per channel. I also have one large toggle to alter the vol. of the subwoofer; but I could have used a fixed resistor instead because I never alter the toggle position. Roederstein 1W are fitted to this toggle.
I have used the shortest possible runs of interconnect (I use Belden 78ohm data cable- BICC electronic cables H8065- reviewed by Martin Colloms years back as being almost as good as the best audiophile cable he then tested). It is inexpensive, and I bought a whole roll and have never yet either looked back or come to the end of it. My interconnect runs are much less than 1 metre from signal to passive and less than a metre from passive to amps. I say I am tri-amped but I actually use 4 amps; 2 monoblocks, 1 stereo amp and 1 stereo amp bridged to mono.
Everything within the passive is hard- wired using silver solder and, mostly, 99.99% pure silver wire -where necessary sleeved in Teflon tubing. I made certain that all resistor leads, switch terminals and wires were squeaky clean before soldering.
The total cost of this passive preamp is low; compare the price of miniature gold- plated toggle switches with gold - plated rotary ones.
What next? I'm thinking of quad-amping. I needed a buffer stage to cope with the added interconnect that tri-amping brought, but, again, this was a compromise worth compromising for. I am also thinking of moving speakers- no not shifting them to a new location- but speaker ENCLOSURES that MOVE (while the music plays).
Eguth[/SIZE[/[/COLOR][/SIZE]COLOR[/COLOR]]][/FONT][/SIZE][/FONT][/COLOR][/SIZE]
it would look a little less heath Robinson 