There are very nice relay controlled input selector/volume control boards with remote control on Aliexpress. With bistable relays even for 64 bit volume control.
If you add a sturdy low Zout preamp PCB with gain the main challenge is encasing the boards.
This is shortest line from A to B in highest possible quality at moderate cost. This front end will be neutral and not have influence on sound character nor will it pick up noise and hum.
If the superfluous stuff then is sold you will have money left instead of spent. And the bonus of good sound without issues. As usual: Less = More.
If you add a sturdy low Zout preamp PCB with gain the main challenge is encasing the boards.
This is shortest line from A to B in highest possible quality at moderate cost. This front end will be neutral and not have influence on sound character nor will it pick up noise and hum.
If the superfluous stuff then is sold you will have money left instead of spent. And the bonus of good sound without issues. As usual: Less = More.
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I've basically got that now with the Schiit Saga S.
I'll have that with the DRV134 board. Gain of 2 which is as much gain as I need.
Doing a test into 4 ohms the amp puts out 22 watts at 400Hz.
Except that the Saga has no gain. You miss the point that it are 2 separate devices with interlinks and added error possibilities for no real reason. Maybe not optimized or simply not optimal when used with eachother impedance wise, gain wise etc. Who knows? It may be solving an imperfection with another imperfection but that seems to be your thing so all is good.
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Everything should work well together but reality sometimes is a bitch. The more stuff is combined the more things can be hampered.
Running in circles. You sure know how to build in possible drawbacks touching the precious signals going to that ruler flat mono amplifier.
Running in circles. You sure know how to build in possible drawbacks touching the precious signals going to that ruler flat mono amplifier.
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The hum was largely from the bias supply in the amp. The only issue is a barely audible rushing sound which is coming from the tube buffer.
The DRV134 board should take care of that.
The DRV134 board should take care of that.
Concerning the amp is there any reason why I cannot replace the driver and phase splitter with this driver and phase splitter or is there a better phase splitter? I'd likely use a 12AX7 if the phase splitter can be modded for it.
The way the driver and phase inverter was originally done was like this.
The reason it was changed.
Also to further balance the 12AX7 plate signals and increase gain slightly.
However looking at this schematic shows it how I changed it. That said it is for a guitar amp so if the circuit did in that amp what it does in my amp it likely made the amp less clean which meant more would be sold given the last thing most guitar players want is a clean sounding amp.
So I will move the 10k resistor tomorrow to where it goes to ground and see what I get.
The reason it was changed.
Also to further balance the 12AX7 plate signals and increase gain slightly.
However looking at this schematic shows it how I changed it. That said it is for a guitar amp so if the circuit did in that amp what it does in my amp it likely made the amp less clean which meant more would be sold given the last thing most guitar players want is a clean sounding amp.
So I will move the 10k resistor tomorrow to where it goes to ground and see what I get.
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A long overdue update.
I got the amp sounding the best I can for now. I do need to get a matched quad of 6V6 tubes though.
Here's the schematic as of now. Gain is set to the stock value of 1 volt for full output which now is 25 watts.
I altered the compensation cap value and followed this procedure to get it set right.
1. Connect scope channel 2 to amp input.
2. Connect channel 1 across 4 ohm non-inductive load.
3. At 1kHz set both channels to where the sinewaves are equal in amplitude.
4. At 20kHz adjust compensation cap to where the sinewaves are once again equal in amplitude.
I noticed the upper treble improved some as the compensation cap I used previously had made it to where at 20kHz the output could not be properly adjusted following the procedure I used which likely was due to increasing the gain of the amp.
I find it interesting how by doing a few things I am able to get a clean 25 watts out of the amp which is 5 more watts than Magnavox spec'd the amp at. That said I don't know at what distortion level the 20 watts was at so I either improved the amp some if 20 watts was 0 distortion or a lot of the 20 watts was at some percent of distortion.
I took my Schiit Sys two input volume control and made a summing box out of it.
1. Shorted input 2 right and left channels together.
2. Connected a 100 ohm resistor from input 1 right channel to input 2 and input 1 left channel to input 2.
I chose 100 ohm resistors to provide a minimum signal drop which from what I measured is only 1mV.
I now have a mono output on input 2 and a variable mono output on the output. I turned on the active mode of the Schiit Saga S since it has a 75 ohm output impedance which will be better for driving the summing box with its 100 ohm resistors.
With the Schiit Saga S set to minimum volume the audio is barely audible from the speaker which is good.
Also given I no longer need the tube buffer I had to reverse the + and - input leads to the speaker crossover given the buffer inverted the signal.
I knew if I worked at it long enough I'd be able to reduce the complexity to where all I needed was the preamp and the passive summing box.
The only minor issue is I have a slight variable noise in the speaker for awhile after the amp is turned on. It is almost like there's a loose connection somewhere.
For a 9.8mVrms input to the amp a 400Hz tone is moderately loud and music is at a decent listening level.
Decided to do a test and a MM phono cartridge connected to the amp will drive it to a decent output level.
So I figure my speaker is quite efficient which could explain why I had to get the power supply ripple voltage as low as reasonably possible to eliminate as mkuch hum as possible.
Concerning the DRV134 board I'll install it in the box the tube buffer is in and use it for when I need to convert an unbalanced signal to a balanced signal. Most likely I will use it in my main stereo as I have a USB audio interface which has balanced inputs and my unbalanced sources are near the rear of my 12' X 12' building while the sound card is near my PC at the front of the building. That way I can run a balanced cable and not worry about any hum being picked up by the cable.
I got the amp sounding the best I can for now. I do need to get a matched quad of 6V6 tubes though.
Here's the schematic as of now. Gain is set to the stock value of 1 volt for full output which now is 25 watts.
I altered the compensation cap value and followed this procedure to get it set right.
1. Connect scope channel 2 to amp input.
2. Connect channel 1 across 4 ohm non-inductive load.
3. At 1kHz set both channels to where the sinewaves are equal in amplitude.
4. At 20kHz adjust compensation cap to where the sinewaves are once again equal in amplitude.
I noticed the upper treble improved some as the compensation cap I used previously had made it to where at 20kHz the output could not be properly adjusted following the procedure I used which likely was due to increasing the gain of the amp.
I find it interesting how by doing a few things I am able to get a clean 25 watts out of the amp which is 5 more watts than Magnavox spec'd the amp at. That said I don't know at what distortion level the 20 watts was at so I either improved the amp some if 20 watts was 0 distortion or a lot of the 20 watts was at some percent of distortion.
I took my Schiit Sys two input volume control and made a summing box out of it.
1. Shorted input 2 right and left channels together.
2. Connected a 100 ohm resistor from input 1 right channel to input 2 and input 1 left channel to input 2.
I chose 100 ohm resistors to provide a minimum signal drop which from what I measured is only 1mV.
I now have a mono output on input 2 and a variable mono output on the output. I turned on the active mode of the Schiit Saga S since it has a 75 ohm output impedance which will be better for driving the summing box with its 100 ohm resistors.
With the Schiit Saga S set to minimum volume the audio is barely audible from the speaker which is good.
Also given I no longer need the tube buffer I had to reverse the + and - input leads to the speaker crossover given the buffer inverted the signal.
I knew if I worked at it long enough I'd be able to reduce the complexity to where all I needed was the preamp and the passive summing box.
The only minor issue is I have a slight variable noise in the speaker for awhile after the amp is turned on. It is almost like there's a loose connection somewhere.
For a 9.8mVrms input to the amp a 400Hz tone is moderately loud and music is at a decent listening level.
Decided to do a test and a MM phono cartridge connected to the amp will drive it to a decent output level.
So I figure my speaker is quite efficient which could explain why I had to get the power supply ripple voltage as low as reasonably possible to eliminate as mkuch hum as possible.
Concerning the DRV134 board I'll install it in the box the tube buffer is in and use it for when I need to convert an unbalanced signal to a balanced signal. Most likely I will use it in my main stereo as I have a USB audio interface which has balanced inputs and my unbalanced sources are near the rear of my 12' X 12' building while the sound card is near my PC at the front of the building. That way I can run a balanced cable and not worry about any hum being picked up by the cable.
Tube Radio said... ''I find it interesting how by doing a few things I am able to get a clean 25 watts out of the amp which is 5 more watts than Magnavox spec'd the amp at. That said I don't know at what distortion level the 20 watts was at so I either improved the amp some if 20 watts was 0 distortion or a lot of the 20 watts was at some percent of distortion.''
Cool news. Nice when a plan comes together. When you did your power computation was it from the scope voltage reading or from a DMM?
Cool news. Nice when a plan comes together. When you did your power computation was it from the scope voltage reading or from a DMM?
I used a scope to view the waveform across the 4 ohm load and put my DMM there to measure the voltage when the sinewave was right below the point of distortion. I used ohms law to calculate the power. I used a frequency of 400Hz.
Do you still have the voltage reading from the scope? Most DMM's lose accuracy as the frequency rises and they have a limit. Did you convert the scope voltage to RMS to see if it confirmed the DMM?
I don't trust my scope for voltage readings as it hasn't been calibrated in years. I do know however that my DMM is good at 400Hz as I didn't see any voltage change from 60Hz to 400Hz.
Sounds good. I should sweep mine on all the ranges just to confirm the book numbers. 500Hz on the lowest range, 1kHz on all the others.
One piece of equipment I need to add is a true RMS AC voltmeter as I use one at work and it even has a nice dB function built in with a relative dB function. That would be good as I could then do a frequency response test and see how flat the amp really is in dB which is much easier than looking at voltages.
I should also get a phase angle voltmeter as that way I can easily see the phase shift of an amplifier instead of depending on my eyes to see it on a scope.
I once used one of those at work to test an amplifier kit I built and it was nice as all I had to do was feed the generator out to the reference in and connect the signal in across the load and get an exact phase angle number instead of guessing based on two scope traces.
I should also get a phase angle voltmeter as that way I can easily see the phase shift of an amplifier instead of depending on my eyes to see it on a scope.
I once used one of those at work to test an amplifier kit I built and it was nice as all I had to do was feed the generator out to the reference in and connect the signal in across the load and get an exact phase angle number instead of guessing based on two scope traces.
I still hear a slight crackling noise with my ear right by the speaker until the amp has been ran for awhile.
Given the gain of the amp is 11.7 and I'm using a rather efficient speaker that would make even the smallest noise in the amp audible.
So I wonder if there's a slightly loose tube socket pin or a cold solder joint or maybe one of the tubes is the problem.
I can test the tubes in my EICO 667 tube tester to see if there's any issues with them.
I still will eventually get a matched quad of 6V6 tubes. I'd prefer NOS unless there's a brand of currently produced 6V6 that will have the same power output and last as long as the NOS 6V6 tubes.
Given the gain of the amp is 11.7 and I'm using a rather efficient speaker that would make even the smallest noise in the amp audible.
So I wonder if there's a slightly loose tube socket pin or a cold solder joint or maybe one of the tubes is the problem.
I can test the tubes in my EICO 667 tube tester to see if there's any issues with them.
I still will eventually get a matched quad of 6V6 tubes. I'd prefer NOS unless there's a brand of currently produced 6V6 that will have the same power output and last as long as the NOS 6V6 tubes.
Try tapping about lightly with a sharpie or similar small insulated stick. Tap lightly on the chassis in different places, around the components and the sockets, even gently tap the tubes to see if they are microphonic. Wiggle the tubes gently and see if the pins are making noise. You'll find it.
I either didn't have the problem until the gain of the amp was increased or the problem was there and the lower gain kept it from being audible.
The amp went from needing about 2Vrms for full output to 1Vrms for full output.
The amp went from needing about 2Vrms for full output to 1Vrms for full output.
It sure is a long way to have normal sound with that mono tube amplifier. Everything around it needs extra stuff or modification. System choices...
The anp indeed needed some work.
I could have kept the amp stock and had poorer sound quality as a result.
That said this amp was made back in the earlier days of HI-FI and was likely very good sounding when compared to other similar amplifiers of the day.
I could have kept the amp stock and had poorer sound quality as a result.
That said this amp was made back in the earlier days of HI-FI and was likely very good sounding when compared to other similar amplifiers of the day.