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Add presence to current feedback on Hitachi Power amp

Started by mladenu, April 26, 2015, 04:53:39 PM

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mladenu

This schematic is from Sessionette Rockette 30 amp, and you will recognize standard Hitachi power amp design.... With wholehearted help from J M Fahey, i add current-feedback to PA which is drawn on picture.. But, i want to add Presence to control CF...

Help me to do that...
Thanks in advance!


teemuk



Loudthud

#3
I would siggest that the cap go to ground so that it reduces the feedback from both Current and Voltage sources. Also there needs to be a limit to the reduction in feedback. Perhaps a 47 Ohm resistor in series with the cap. The best way to look at this is with simulation. Less smoke that way  8)

J M Fahey

You can't clap and play the piano at the same time ... or even if you can, you'll do it poorly.

I also had that problem of needing a presence control in a mixed feedback amp (or straight current feedback).

After wasting much time experimenting and getting only so-so results, I bit the bullet and added a presence stage at the end of the preamp.

As simple as adding a pot/cap/resistor at the last stage NFB *or* , if signal level was high enough (it often is)  I added a passive one.

Now if the amp is a regular voltage feedback one, you *can* add it the standard way, but often the pot value is a problem, with average amps you need a 1k pot, maybe 2k, hard to find.

mladenu

 :( I hope that some of you guru`s, specialy you, J M Fahey, have solution, but my "My ships had sunk".. :( Firstly when i saw this post: http://www.diystompboxes.com/smfforum/index.php?topic=81956.msg679661#msg679661 hope is born... but apparently in vain... Write me something.. Is it straight or mixed feedback in my Hitachi power stage? Thanks for your answer and help, also all of you guys are awesome!

J M Fahey

As drawn, your amp is mixed feedback.

Why "mixed"?

Because you have 2 feedback voltages in series, entering Tr2 base (which is NFB injection point):

1) 33k/470r = 70X  so if you have, say, 20V at the speaker out, the *voltage*  dependent NFB voltage is 20000mV/70=286mV

2) supposing an 8 ohms speaker the voltage divider is: 8r/0.22r=36.4X and the NFB voltage based on output current is: 20000mV/36.4=549mV

Both add up because they are in series (R5 is in series with the 0.22r ceramic one) so NFB voltage is: 286+549=835mV

So that is also the power amp input sensitivity: for 20V@8 ohms (50W RMS) it needs some 835mV at the input.

For full power it needs around 1V RMS , which is the unwritten standard.

By the way, don't worry about what an LM386 can not do ;)  , I'm amazed at what it can, given it's such a crude tiny amp, but don't extrapolate from it to "real" amps.


Roly

Awwww... that is so cute, an LM386 with current feedback!   <3)



Lookit the lil' fella, pretending to be a Marshall stack.   :lmao:


{when these were new and came in round cans I tried quite a few things including power oscillator and compressor/limiter; useful but the signal quality isn't great.}
If you say theory and practice don't agree you haven't applied enough theory.

mladenu

#8
Perfectly clear! As always J M Fahey.. :) But, how can i mod my power amp to have straight current feedback?

I am also in consideration to build lm3886 with current feedback.. :/ Of course, guitar amp... :) Do you, or some of you, have some simple circuit for my need?

Thanks guys...

Roly

Quote from: mladenuhow can i mod my power amp to have straight current feedback?

{This is JMF's dance, but I'll just remark that} I don't see how you can remove the voltage feedback.

These s.s. power output stages are like power op-amps, they have very high open-loop forward voltage gain, let's guess at 50dB.  They apply about 30dB of Negative FeedBack to reduce the working voltage gain to about 20dB.

Just as the open loop gain of an op-amp without NFB would make it unworkable as a linear amplifier, so if you remove the voltage NFB from a power stage like this the input sensitivity will rise to the open loop figure, i.e. by 30dB to 50dB, or around 30 times greater.

As it is;

Quote from: J M FaheyFor full power it needs around 1V RMS , which is the unwritten standard.

So the sensitivity without VNFB would be;

1/30 = 0.03333333 or 30mV for full output.

But it's not just input sensitivity, the VNFB also reduces distortion by 30dB, significantly flattens the bandwidth, and prevents instability.  Without VNFB it is quite likely that the amp will idle with the output at one of the supply rails (across the speaker!) because there is now no mechanism for it to rebalanced the amp output to the zero volt reference at the input (the diff pair stage TR1/2).  Thermal drift will no longer be compensated.

The idea behind current feedback in s.s. amps is this;

- valve amps have a high source impedance to the speaker, in the order of ohms, so the dynamic damping of the speaker cone is low and the speaker is free to colour the sound.

- s.s. amps have a very low source impedance, small fractions of an ohm (in part due to the very high levels of NFB used) and so provide maximal dynamic damping to the speaker cone.

The effect of applying current feedback is that it causes the output impedance to be higher and give less dynamic damping, hopefully giving more of a "valve sound".

By the nature of s.s. output stages this must be an add-on to voltage feedback, and cannot substitute for it.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Just leave as-is.

Now you have about 2/3 current feedback and 1/3 voltage feedback; as Roly noted you still need at least a little voltage feedback so you are at the practical limit.

More important, you probably already have about 90% of the sound you *might*  have, in theory, which in practice is excellent.

mladenu

That`s all i wanted to hear (read).. :) I do not touch anything, promise! :D

But, i wanna try to make Amp camp amp for guitar purpose... PhabbTone in front of Nelson Pass project.. Guess what confusing me.. How to add current feedback in this topology? :) I saw some of ideas which may solve my questions, but i trust only to you guys..  :tu: Thanks all of you for your patience and good will to help me..

Here is a schematic for Nelson Pass ACA amp, and info link...

http://www.diyaudio.com/forums/diyaudio-com-articles/214808-amp-camp-amp-1-a.html



Loudthud

In this thread a builder tried mixed mode feedback but later abandoned the idea.

http://www.diyaudio.com/forums/pass-labs/248030-aca-5w-guitar-power-amp.html

The ACA has an output impedance of several Ohms when the feedback is disconnected so it sounds just fine as a guitar amp.

J M Fahey

Agree and add: I respect Mr Nelson Pass very much, but he delves in another world, where a 5W single ended triode amp, fed 1250V and needing a silver wire wound transformer is the reference, so his 5W amp is excellent, since it sounds very close to the tube one.

But for **us** ?

No way José.

That hot and heavy Class A amp can not compete with a drummer, period, which is *our*  reference, like it or not.

FWIW with 4 x IRFP240 I easily get over 300W RMS in my Bass amps, which stand grueling 8 hour live sessions at Stadiums .... here he gets 5W out of 2 of them, clearly a different way to do things.