To start, I'm not trying to steal Mr. Pass's idea for a transconductance amplifier. Of course, my implementation is far different from his to begin with.
I've begun developing a transconductance amplifier for my own personal use and thought I'd like to talk about it.
I'm using a three-stage design with error amplifier, so this does have feedback, and is more like a typical three stage voltage amp than anything. The major difference is that the feedback loop is now with a speaker in place of the feedback resistor, so that it senses current drawn through the speaker.
For the prototype, I'm using an op-amp to drive a push-pull output stage. This is done through an emitter follower which is loaded by a current source to drive the output transistors. Nothing new here.
My desire to do this was merely out of wanting to see how a transconductance amp acted. I'd never used one for driving loudspeakers before.
To say the least, the results are absolutely astonishing. As I have studied, the effects of a lot of internal speaker limitations are more or less taken care of. It really is something else.
If anyone would like to see what I've done exactly, I suppose I could post a schematic.
I've begun developing a transconductance amplifier for my own personal use and thought I'd like to talk about it.
I'm using a three-stage design with error amplifier, so this does have feedback, and is more like a typical three stage voltage amp than anything. The major difference is that the feedback loop is now with a speaker in place of the feedback resistor, so that it senses current drawn through the speaker.
For the prototype, I'm using an op-amp to drive a push-pull output stage. This is done through an emitter follower which is loaded by a current source to drive the output transistors. Nothing new here.
My desire to do this was merely out of wanting to see how a transconductance amp acted. I'd never used one for driving loudspeakers before.
To say the least, the results are absolutely astonishing. As I have studied, the effects of a lot of internal speaker limitations are more or less taken care of. It really is something else.
If anyone would like to see what I've done exactly, I suppose I could post a schematic.
I suspect this will have been done before i can think of a few examples but i would like to see a schematic values dont have to be shown!
Then i will be prepared to pass opinions / comments
regards trev
Then i will be prepared to pass opinions / comments
regards trev
Duo said:
About 15 years ago, in the beginning of the 90-ies, I think,
Elektor had such an amplifier.
I could find this article and post a copy of schematic.
I have all them old Elektor magazines stored here.
The basic and rather simple circuit can be seen in my attachment.
I have drawn this schematic from memory.
It shows the Current Feedback princip.
The Gain in this case is: LSP 8 Ohm / 1 Ohm = 8
Regards
lineup
Attachments
ive done that with an lm3875..
the results were pretty good, super bass but i found the mids to be extremely hard and the there seemed to be a rising highs, i think it is because of the rising impedance curve of the single driver.
all it required abit of a filter and it was pretty nice.
the results were pretty good, super bass but i found the mids to be extremely hard and the there seemed to be a rising highs, i think it is because of the rising impedance curve of the single driver.
all it required abit of a filter and it was pretty nice.
there is a more efficient way by having 2 feedback loops and the sense resistor at the output, that way you wont have a floating output problem.
hacknet said:
Yes,
this is probably the way to do it, hacknet.
My attachment is not a complete design circuit.
As I told, it is a simplified diagram - to show the basic princip.
Actually this is what I would call A True Current Feedback Amplifier.
CFA - In the correct sense.
Because gives an output current proportional to input voltage.
🙂
One thing about woofers/speakers to use with this.
You may have to rolloff the rising impedance of the speaker.
Using some impedance equalizing filter.
Similar in effect to the Zobel Network.
Say the impedance would rise to 30 Ohm in higher freq.
The gain would be also 30!!
Regards, lineup
lineup said:
Don't know that much about amps, but I think I might have caught a small error here.
If the normal impedance of the speaker is 8 ohms, and it gradually rises to 30 ohms, wouldn't the gain be 30 / 8 = 3.75?
Which would still be considerable, and very hearable, but not as high as if the gain were 30.
kelticwizard said:
See my attachment:
http://www.diyaudio.com/forums/attachment.php?s=&postid=1118549&stamp=1170003209
The gain is: LSP-Z / R3
R3 is in this case 1 Ohm.
So, Nominal gain is: 8/1 = 8
At Z=30 Ohm ...... 30/1 = 30
This is why we need to watch out, that speaker impedance stays within a reasonable level.
Nelson Pass, for his First Watt amplifiers, suggested simply a resistor in parallel with the Loudspeaker.
It will work.
--------------------------------------------------------------------
But I prefer use a Impedance Equalizing Network:
Le is the inductance of your driver's voice-coil (in henries).
Re is the D.C. resistance of the voice-coil (ohms).
The design equations for this type of filter are:
Capacitor = Le/Rc2
Resistor (Rc) = 1.25 X Re
Websource:
Zobel Loudspeaker Impedance Correction Circuit
I have seen something like this in the 1970,s I think a company called servosound did this allthough in that case it was a mixture of both voltage and current feedback !
I have also seen H+H amplifiers use a degree of current feedback with very good results
Rod Elliot The Ausie has an article that relates to this on his web site !
Always worth a visit
Just remember when dealing with the amplifier spk combination a moving coil speaker is not only a varying impedance but can also be a generator ie a negative impedance such as at times when the cone/coil overshoots and for this reason I believe your ampifier may excert better control
regards Trev
I have also seen H+H amplifiers use a degree of current feedback with very good results
Rod Elliot The Ausie has an article that relates to this on his web site !
Always worth a visit
Just remember when dealing with the amplifier spk combination a moving coil speaker is not only a varying impedance but can also be a generator ie a negative impedance such as at times when the cone/coil overshoots and for this reason I believe your ampifier may excert better control
regards Trev
lineup said:
Actually, the reason I paralleled a resistor with the speaker was
to provide a lower output impedance for the loudspeaker -
there are very few drivers that are optimal with source impedances
greater than 15 ohms or so.
😎
It may take me a while to get around to a schematic.
At higher output, the circuit is almost impossible to stabilize, probably due to the high open loop gain and the amount of feedback I have. I definitely will be trying all discrete so I can trim things down a bit more.
I'll slowly chop at it and get back to you.
At higher output, the circuit is almost impossible to stabilize, probably due to the high open loop gain and the amount of feedback I have. I definitely will be trying all discrete so I can trim things down a bit more.
I'll slowly chop at it and get back to you.
Duo said:
Current feedback amplifiers can be a bit tricky to get stable.
I remember I had oscillations when setting up that Elektor circuit.
Current feedback amps can be very fast - high slewrate ...
Usually not a good idea, to have same supply line
for input, pre stage and output stage.
Actually not uncommon to see Current Feedback amplifiers
with separate regulated supply for the front end part!
Heh, I have tried separate regulation and filtering for the opamp besides the output stage.
I think the deal is just that I need to get the opamp out of there, or put a better one for this application in place.
I know I could probably stabilize it, but I'd rather try alternative methods which are just more stable.
I think the deal is just that I need to get the opamp out of there, or put a better one for this application in place.
I know I could probably stabilize it, but I'd rather try alternative methods which are just more stable.
It seems that with some slight modifications, this is now stable; with the same op-amp I started with.
I had to add some B-C capacitance in the current source that operates the pulldown for the op-amp's emitter follower to drive the outputs. This slowed it down enough to stay stable up to higher output than I originally intended. I'm now happy with the circuit for the most part.
I had to add some B-C capacitance in the current source that operates the pulldown for the op-amp's emitter follower to drive the outputs. This slowed it down enough to stay stable up to higher output than I originally intended. I'm now happy with the circuit for the most part.
UPDATE: The LF411 op-amp co-operates with this topology beautifully.
It's absolutely stable in situations where my other op-amp would not calm down. I was using an MC4558CP1 dual op-amp.
I would be picking much better op-amps if I could, but I'm doing this project with only the parts I have around in my junk collection, so bear with me.
I'm pleased enough with this one I may built the first prototype with it.
It's absolutely stable in situations where my other op-amp would not calm down. I was using an MC4558CP1 dual op-amp.
I would be picking much better op-amps if I could, but I'm doing this project with only the parts I have around in my junk collection, so bear with me.
I'm pleased enough with this one I may built the first prototype with it.
Nothing more than a star ground. There are only a few things needing a ground connection and they are connected to a point, instead of a rail.
I haven't had any grounding issues so far.
I haven't had any grounding issues so far.
while working on an amp design of my own, i tried using a combination of voltage and current feedback. i had read a paper available online from www.national.com about the "alexander amp"...... my amp got very unstable as well..... the problem with the design i've been working with is that b-c compensation in the VAS seems to have no effect at all, except to change the oscillation frequency....... and after further experimentation, i've discovered that it's the VAS itself that is unstable, so i need to work on it....... back to the question at hand, a CFB amplifier sounds great, and i'll get the URL for you of that paper from N/S.....
but not right now...... gotta new job, been working all day, gotta get up in the morning, and my mind can't take the excess bias current at the moment...... L8R......
but not right now...... gotta new job, been working all day, gotta get up in the morning, and my mind can't take the excess bias current at the moment...... L8R......
- Status
- Not open for further replies.