Audio circuit design for your comments and advises

[pha0001m]

Hi guys, here is my design draft for a hybrid audio amplifier.

The design has three stages.

1. An input buffer stage.
2. A differential pair phase splitter stage.
3. The power stage/output stage.

The input audio signal will come from an Iphone, and I assumed that an Iphone can output 1V audio signal. Thus, I designed the close loop gain to be 20, with 18dB of negative feedback.

Any comments or advises regarding this amplifier will be greatly appreciated

 

[chuckey]

Have you got the output transformer sorted out, I suspect it will cost $$$$$
Frank

 

[pha0001m]

Have you got the output transformer sorted out, I suspect it will cost $$$$$
Frank

Yup, already got myself one of this guy. They are quite cheap actually, and plus the toroidal transformer has an electrostatic shield(which is quite rare for toroidal transformer). I have not test it out yet, but hopefully it will behave .

 

[pha0001m]

As for the output transformer, I will get the Hammond 1650FA
https://www.digikey.com.au/product-search/en/transformers/audio-transformers/786564?k=1650FA

 

[betwixt]

It's certainly an interesting mix of technologies!

I suspect both transformers will cost $$$$$.

As far as I can tell it should work fine although I have reservations about it's cost effectiveness when an all solid-state design could produce equal or better results at a fraction of the cost.

A suggestion - why not try a constant current generator instead of the cathode resistor. In theory the current is constant anyway because one 6CA7 will counter-balance the other but it would be interesting to see if actually forcing it to be constant would help, particularly as they start to age.

Brian.

 

[FvM]

Shouldn't the thread be posted in the Tubes and Retro section? :smile:

Besides other points that have been mentioned, I'm not sure if the LT3080 will survive under all possible switch-on transients. I would provide zener clamping diodes for the set terminals to be on the safe side.

 

[Audioguru]

How often will you replace the worn out or burnt out vacuum tubes??

 

[pha0001m]

It's certainly an interesting mix of technologies!

I suspect both transformers will cost $$$$$.

As far as I can tell, it should work fine although I have reservations about it's cost effectiveness when an all solid-state design could produce equal or better results at a fraction of the cost.
Brian.

An all out solid-state is indeed more superior both in term of linearity and power efficiency. This is a fact which I won't deny. In audio system however, sometime a little distortion is preferred, especially in guitar amplifier, where the soft and gradual clipping of the tube is desired when the amp is driven into saturation, unlike transistors, they tend to saturate very abruptly. Aside from the electrical aspect, I personally perfected tubes for their glow and glassy look.

Power efficiency however is poor for traditional valve amp. This is why I decided to implement a mix technology, and removed all the power wasted in the pre-amp and rectifying stage found in traditional tube amp(2/3 of the wasted power), while still retain the traditional tube sound.

It's certainly an interesting mix of technologies!

I suspect both transformers will cost $$$$$.

A suggestion - why not try a constant current generator instead of the cathode resistor. In theory the current is constant anyway because one 6CA7 will counter-balance the other but it would be interesting to see if actually forcing it to be constant would help, particularly as they start to age.

Brian.

A constant current generator is a top idea! I tried it in LTspice, and it worked. I will give it a ago when the time come. I suspected that by having a constant current sink there will greatly help improve the PSRR and CMRR of the stage. It properly will also help force the bias current to be stable as the tube age.

Shouldn't the thread be posted in the Tubes and Retro section? :smile:

Besides other points that have been mentioned, I'm not sure if the LT3080 will survive under all possible switch-on transients. I would provide zener clamping diodes for the set terminals to be on the safe side.

FvM, I thought it would be more appropriate to post it here, since there are not much of a valve concept here. The majority of the design is solid-state.

Zener clamping diode across the LT3080 sound pretty good for me. To be honest, I think getting this power supply to survive during turn on transient will be the most difficult part of this project. I hope that the relay current limiting concept on the power supply will take care of this problem.

How often will you replace the worn out or burnt out vacuum tubes??

I have never own any tube amps, this will be my first if I can successfully build it, but a quick search on Google pointed to some where between 2 to 5 years with moderate usage.

 

[Audioguru]

My first amplifier was a kit and it used vacuum tubes. I thought it sounded pretty good when it was about 3 months old. Then a high-end amplifier manufacturer set up a clinic in the local audio equipment store and tested your amplifier to show that theirs was much better. Mine tested awful, with high distortion. I replaced the 3 months old tubes and the distortion vanished but returned a few months later as the tubes wore out and became unmatched. I sold the amplifier and bought a solid state receiver that still works perfectly today, 50 years later.

Some rock musicians replace their vacuum tubes before every show.

 

[pha0001m]

My first amplifier was a kit and it used vacuum tubes. I thought it sounded pretty good when it was about 3 months old. Then a high-end amplifier manufacturer set up a clinic in the local audio equipment store and tested your amplifier to show that theirs was much better. Mine tested awful, with high distortion. I replaced the 3 months old tubes and the distortion vanished but returned a few months later as the tubes wore out and became unmatched. I sold the amplifier and bought a solid state receiver that still works perfectly today, 50 years later.

Some rock musicians replace their vacuum tubes before every show.

If my EL34 happen to last for only three months, I will follow the same path as you! :laugh:

This is a big problem with vacuum tubes, as they age, they tend to move all bias points. Most traditional tube amps required at-least four additional tubes for the pre-amp, driver and power rectification stages. All of which will be affected as the tube aged.

I hope that by replacing all of these stages with solid stage, the problem would be lessen as there will be less tubes in the system, while still retain the traditional sound of tubes.

 

[Audioguru]

Tubes clip gracefully, solid state clips horribly. Simply do not allow a solid state amplifier to clip. Do it with more power than you need.
Tubes are not linear, they produce even harmonic distortion that some people say is musical. But distortion is something that should not be there.
The output transformer in a tube amplifier messes up the frequency response and provides poor damping of the resonances of speakers.