Hi Adolph,
Yes that was all I did. While they were on crocodile clips I tried two pairs of caps (Jupiters and stock polyesters) and on each pair I could create the oscillation depending on whether I went from section 1 of the first tube to section 1 on the second tube (no oscillation) or section 1 to section 2 (oscillation returned).
- I just ran a mono signal into the amp from a signal generator and measured the output of the amp to check on the original balance issue and both outputs seem to be pretty well matched at the volume settings I tried.
I am just a bit concerned about the 120mv DC at the speaker outputs now. Perhaps this was there before, I'm not sure.
Is this something that can be improved upon without a huge amount of work?
Yes that was all I did. While they were on crocodile clips I tried two pairs of caps (Jupiters and stock polyesters) and on each pair I could create the oscillation depending on whether I went from section 1 of the first tube to section 1 on the second tube (no oscillation) or section 1 to section 2 (oscillation returned).
- I just ran a mono signal into the amp from a signal generator and measured the output of the amp to check on the original balance issue and both outputs seem to be pretty well matched at the volume settings I tried.
I am just a bit concerned about the 120mv DC at the speaker outputs now. Perhaps this was there before, I'm not sure.
Is this something that can be improved upon without a huge amount of work?
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Ahh... So that 'splains a lot. You had your channels crossed apparently. Thus the feedback signals were crossed. Should spell trouble.
I wouldn't be too worried about that DC offset. The amp has cathode feedback, so all the power stage current flows through the secondary side. And since this isn't exactly zero ohms, it will cause a small voltage over it. If the power stage is biased right, I wouldn't worry.
I wouldn't be too worried about that DC offset. The amp has cathode feedback, so all the power stage current flows through the secondary side. And since this isn't exactly zero ohms, it will cause a small voltage over it. If the power stage is biased right, I wouldn't worry.
Thanks for the reassurance Adolf.
Well I finally got it back into the system.
No oscillation, no imbalance, no hum! SUCCESS!
Thank you very much to everyone who has taken the time out to reply to my many questions and posts in this thread.
It sounds so much better than it ever has, especially the balance. The sound stage really wraps around you like it should now. I guess all those new caps will need time to settle in but right from the off I am very happy.
And to think I was very close to giving up!
Well I finally got it back into the system.
No oscillation, no imbalance, no hum! SUCCESS!
Thank you very much to everyone who has taken the time out to reply to my many questions and posts in this thread.
It sounds so much better than it ever has, especially the balance. The sound stage really wraps around you like it should now. I guess all those new caps will need time to settle in but right from the off I am very happy.
And to think I was very close to giving up!
Hi guys. I hope you don't mind that I resurrect this thread. I have Almarro A205A mk ii and Almarro A318B amps both with V-Cap CuTF coupling caps at the front end installed at the factory. I am considering upgrading the 0.22 uF coupling caps in the A318B and their equivalent in the A205. Do you see any problem with using the Jupiter Cooper Foil caps for this application, as they come highly regarded and are significantly less expensive. Thanks.
Hello Gents,
I am also having some issues with the input selector in my Almarro A318B.
I guess I may have missed this info through this thread, But, do any of you guy have the proper dimensions ,shaft length, maker, for aftermarket selectors ?
I was also thinking about replacing the volume pot with a Noble Pot 100K,do any of you guys know if it will fit and or work properly?
Thanks
I am also having some issues with the input selector in my Almarro A318B.
I guess I may have missed this info through this thread, But, do any of you guy have the proper dimensions ,shaft length, maker, for aftermarket selectors ?
I was also thinking about replacing the volume pot with a Noble Pot 100K,do any of you guys know if it will fit and or work properly?
Thanks
Hi everyone,
I'm planning to get Almarro 318B and modify it to power amp use.
Speakers will be horn driver with 107dB/W and 16 ohms nominal impedance.
My idea is to remove input selector and input level adjustment potentiometer both being unnecessary for intended use.
I have tube preamp which can output high voltage levels (up to 30VAC). Therefore low gain of power amp is preferred.
Almarro has output transformer equipped with 16 and 32 ohms tabs. I wish to use 16 ohms tap to drive the horn speakers (16 ohms) to allow maximum output voltage swing.
Is there any reason not to replace 8 ohm tabs with 16 ohms tabs and could/should I then move the NFB from 16 ohm output to 32 ohm output? (there might be a reason to separate speaker load from transformer NFB terminal?)
In that case does the NFB resistor 22R need to be adjusted or something else?
Since low gain is preferred could I just simply use 6SN7 in input stage as well as driver stage? Is it necessary to adjust anode or cathode resistor in that case?
If potentiometer is removed it's required to terminate first stage grid to ground with 250k resistor? That would then present 250k input impedance.
Any other ideas to optimize SQ and keep gain as low as possible (lets say reasonably low)?
Also low frequencies will not be produced with this amp (crossover @200Hz) so could 0.47uF or 0.22uF or both interstage coupling caps be reduced in value to high pass amp from unnecessary signal amplification?
As you can see, I'm not expert with tube amps but I have years of experience working with low voltage AC/DC equipment.
I'm planning to get Almarro 318B and modify it to power amp use.
Speakers will be horn driver with 107dB/W and 16 ohms nominal impedance.
My idea is to remove input selector and input level adjustment potentiometer both being unnecessary for intended use.
I have tube preamp which can output high voltage levels (up to 30VAC). Therefore low gain of power amp is preferred.
Almarro has output transformer equipped with 16 and 32 ohms tabs. I wish to use 16 ohms tap to drive the horn speakers (16 ohms) to allow maximum output voltage swing.
Is there any reason not to replace 8 ohm tabs with 16 ohms tabs and could/should I then move the NFB from 16 ohm output to 32 ohm output? (there might be a reason to separate speaker load from transformer NFB terminal?)
In that case does the NFB resistor 22R need to be adjusted or something else?
Since low gain is preferred could I just simply use 6SN7 in input stage as well as driver stage? Is it necessary to adjust anode or cathode resistor in that case?
If potentiometer is removed it's required to terminate first stage grid to ground with 250k resistor? That would then present 250k input impedance.
Any other ideas to optimize SQ and keep gain as low as possible (lets say reasonably low)?
Also low frequencies will not be produced with this amp (crossover @200Hz) so could 0.47uF or 0.22uF or both interstage coupling caps be reduced in value to high pass amp from unnecessary signal amplification?
As you can see, I'm not expert with tube amps but I have years of experience working with low voltage AC/DC equipment.
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Hi! Sorry to bump this old thread. I have an Almarro A318A and I would like to reroute the signal from the preamp to one of the RCA inputs and reroute the power amp in to another one of the RCA inputs. This would allow me to insert a minidsp into the signal between the preamp and power amp. This would leave me with only one rca input.
Basically I would want to make it have a power amp in / preamp out like the ones in in vintage receivers.
My question is would this be possible and what do I have to watch out for in terms of the safety of the amp.
Thanks!
Basically I would want to make it have a power amp in / preamp out like the ones in in vintage receivers.
My question is would this be possible and what do I have to watch out for in terms of the safety of the amp.
Thanks!
You will need to solve two problems.
#1 dsp typically has lower input impedance. The Almarro A318A pre-amp section is designed for ~450K load, not a few 10s of kohms.
#2 The cable and connection capacitance that you will be putting in.
You will have to add a line buffer stage to avoid both output bandwidth and amplitude drop.
#1 dsp typically has lower input impedance. The Almarro A318A pre-amp section is designed for ~450K load, not a few 10s of kohms.
#2 The cable and connection capacitance that you will be putting in.
You will have to add a line buffer stage to avoid both output bandwidth and amplitude drop.
Thank you for this! The minidsp outputs at 2Vrms will this be enough to pass through the power amp in? Again appreciate your answers. Thanks
Here’s the specs of the in/out of the minidsp:
Input / Output Max Levels
IN: 2Vrms / 4Vrms (jumper-selectable)
OUT: 2Vrms
Input Impendance:10 kΩ
Output Impedance: 560 Ω
Thanks for the help!
Here’s the specs of the in/out of the minidsp:
Input / Output Max Levels
IN: 2Vrms / 4Vrms (jumper-selectable)
OUT: 2Vrms
Input Impendance:10 kΩ
Output Impedance: 560 Ω
Thanks for the help!
No, You will need about 6Vrms output to get full power out. Being a dsp, may be you can use a 1X and 3X Op Amp as buffers.
Thanks! Seems I will need buffering before and after the minidsp. Thanks for the help!
Hi! Did a lot of reading for this. Would it be adviseable to build outboard buffers (tubed or non tubed) outside the Almarros to bring up the preamp’s gain and to bring up the minidsp’s gain before it hits the power amp section of the Almarro?
If this is a possible and noiseless solution are there any schematics or ready made kits that can do this outboard buffering?
Thanks!
Hi! Did a lot of reading for this. Would it be adviseable to build outboard buffers (tubed or non tubed) outside the Almarros to bring up the preamp’s gain and to bring up the minidsp’s gain before it hits the power amp section of the Almarro?
If this is a possible and noiseless solution are there any schematics or ready made kits that can do this outboard buffering?
Thanks!
Hi, resurrecting an old thread because I have the same imbalance problem. However, I kind of concluded that the problem is in the volume knob.
When I turn the volume knob, there is no sound until I turn it to about 7o’clock position. Then turn it a bit further there will be broken sound from the left. Then turn it a bit further it have sound on the right channel but not on the left. Turn it a little bit further to about 8 o’clock then there is sound on both channels, and the sound stage is center. Everything seems good. But when I turn further, say to 9 o’clock, the sound then move toWards the left. So, I would like to replace the volume knob, but from earlier post, it seems the size may not fit well. To OP, @moley, what brand of volume knob you use and does it fit?
Second question, if I can’t find something that fits, my Toppings dx7 pro has a volume control. But then it will go through it’s headphone amp section. It has another DAC mode which bypass the headphone amp section and no volume control. Which way is theoretically better? Output in DAC mode and replace the volume know on the Almarro, or use the volume control in Topping and bypass the volume control in Almarro?
When I turn the volume knob, there is no sound until I turn it to about 7o’clock position. Then turn it a bit further there will be broken sound from the left. Then turn it a bit further it have sound on the right channel but not on the left. Turn it a little bit further to about 8 o’clock then there is sound on both channels, and the sound stage is center. Everything seems good. But when I turn further, say to 9 o’clock, the sound then move toWards the left. So, I would like to replace the volume knob, but from earlier post, it seems the size may not fit well. To OP, @moley, what brand of volume knob you use and does it fit?
Second question, if I can’t find something that fits, my Toppings dx7 pro has a volume control. But then it will go through it’s headphone amp section. It has another DAC mode which bypass the headphone amp section and no volume control. Which way is theoretically better? Output in DAC mode and replace the volume know on the Almarro, or use the volume control in Topping and bypass the volume control in Almarro?
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