Fostex Meets Pentode I need some NFB

[Wavebourn]

The good thing about these type of circuits is that they are all in parallel with the driver, so the speakers can still be driven just fine by a voltage source amp

Do you mean a resistor in series with the driver?

 

[Jeb-D.]

Do you mean a resistor in series with the driver?

The resistors, capacitors and inductors are all in parallel with the driver. Check this article
http://www.passdiy.com/pdf/cs-amps-speakers.pdf

I've used similar circuits to drive a full-rangers using Pentode output stages without NFB. It tames the impedance variation of the driver, but it doesn't have to be flattened completely, which is where the benefits come it.

 

[Tweeker]

Into really low impedance loads, 6L6 family makes mostly 2nd harmonic, though lots of it. A differential pair current amp might be interesting.

 

[Wavebourn]

The resistors, capacitors and inductors are all in parallel with the driver.

In such case with a voltage amp (very low output resistance) the single effect of them is increase of power losses.

Check this article
http://www.passdiy.com/pdf/cs-amps-speakers.pdf

Nelson means in the article exactly what I said: you may use such an approach with "voltage amplifiers", but R0 how he calls it should be in series with the load.

I've used similar circuits to drive a full-range using Pentode output stages without NFB. It tames the impedance variation of the driver, but you don't have to flatten it completely, which is where the benefits come it.

A 31 - band graphics EQ is your friend.

 

[Jeb-D.]

In such case with a voltage amp (very low output resistance) the single effect of them is increase of power losses.

That is true. At some frequencies there will be an increase in loss, but power dissipation will be more consistent throughout the frequency range.

Nelson means in the article exactly what I said: you may use such an approach with "voltage amplifiers", but R0 how he calls it should be in series with the load.

Sorry I didn't totally understand your statement at the time. Of course the principal can be adapted to "voltage amplifiers", but both frequency response and impulse response(which will differ with source impedance) must be considered. The point of my posts is that a Pentode can be used to drive a pair of full rangers (really well) without NFB. Not that I'm against NFB, many of my designs use it.

I think the "Pentodes have bad sonics" rap is purely from mis-application. A Pentode amp would probably do really well with Magnepan speakers as well. They have a near flat impedance and require a lot of power!

A 31 - band graphics EQ is your friend.

I've never got to play with a good one, but I bet that is true.

 

[Wavebourn]

I think the "Pentodes have bad sonics" rap is purely from mis-application. A Pentode amp would probably do really well with Magnepan speakers as well. They have a near flat impedance and require a lot of power!

I've never used pentodes without feedback: I believe that both extremes cause most distorted power amplification.

By the way, solid state amps (except probably designed by John Curl) with resistors added in series with speakers sound much better than without them.

 

[planet10]

two identical drivers in series (FE127) with a capacitor in between them to shunt high frequencies to ground, this way, all of the high frequencies goes into the front driver, while the baffle step correcting low frequencies get spit out both drivers.

To get the front driver only to produce highs the cap has to be across the rear driver's terminals... if the cap is in series with both drivers, you are just rolling off the bass.

dave

 

[planet10]

A 31 - band graphics EQ is your friend.

If you can find one that doesn't sound disgusting...

dave

 

[alexmoose]

I read the article about Peter Millet's Might Midget because it uses Plate to Grid feedback, and I read something when I notice a line on page 3 that said "An un-bypassed Cathode Resistor is s form of Cathode feedback"

This got me to thinking, what If I apply local feedback by using a Zobel in series with the bypass Cap? is this practical?

 

[Wavebourn]

If you can find one that doesn't sound disgusting...

I made pair from ancient Altec Lansing LC EQ.

They had initially too many opamps with horrible additional transistor buffers.

 

[planet10]

I made pair from ancient Altec Lansing LC EQ.

When i was making my comment, my thots were that that approach might have a chance...



dave

 

[alexmoose]

Applying cathode feedback (putting the cathode bypass cap to Speaker terminal in stead of ground) gave me ~7.8 db of NFB, I think I am going to try and reduce that to ~3db with a capacitive voltage divider. However Preliminary listening is disappointing, It sounds Kinda like a bad "Sand" amp, distortion is defiantly characteristic of Negative feedback being present. I am going to try a voltage divider to bring that feedback down, but It looks like Zobel is going to be the answer...