Simple Chip Amp for P to P wiring

[coldcathode]

BTW if you remove the cathode bypass cap you drop the gain to about 5V and get some bass response back.

[danielwritesbac]

The typical sound card has either 10k or 100R, not 100k. EDIT: If the output caps are 10uF, its a 10k card. If the output caps are huge, it has a headphone amp onboard. Let's try for the 10k for now. Do we need a Jfet buffer chip?

[danielwritesbac]

Originally Posted by coldcathode

BTW if you remove the cathode bypass cap you drop the gain to about 5V and get some bass response back.

So, for the 6N11, 6922, 6DJ8, and ECC88 it drops the gain (it already has plenty anyway?) and decreases distortion. Is that right? What is the gain after removing the cap?

[ratza]

AndrewT, you're right about phase margin. Although in theory 1 degree should be enough to ensure stability, I'd rather have at least 60.

daniel, you do not need input buffer, because the tube's input impedance is quite big. If you want to see how the calculation is made, take a look here:
Designing Common-Cathode Amplifiers

[danielwritesbac]

Cool. Howabout inverting mode LM3886 to drop the gain?

[ratza]

It will work, the gain will be equal with -R14/R3. However, the input impedance will be set by the value of R3, being 10k. I would remove the R11-C8 and R13-C10 and add a resitor between noninverting input and ground. The value has to be identical with R3 in order to minimize the input voltage offset. Also, 220pF between inputs will help removing pops and clicks.

AndrewT, you clearly are a better engineer than me and I have a question for you. I've read a ton of documentation and I couldn't find an explanation for this. Why the inverting configuration is stable also at lower than unity gain?

[AndrewT]

Originally Posted by ratza

Why the inverting configuration is stable also at lower than unity gain?

I don't know if that is true.
I don't use inverting.
The only place I have used inverting is CFP filtering and second stage of an active RIAA preamp.

[ratza]

I don't know for sure either. If the gain/bandwith/phase diagram applies for both inverting and non inverting configurations, then the LM will oscillate for sure at a gain of 3. I have to test this.

[AndrewT]

btw,
the experts here say that 60degree phase margin is not enough. I originally thought >=45degree prevented oscillation as long as the slope was <=9dB/octave at cross over.

But, there seems to be consensus that 80 to 85degrees phase margin gives best reproducibility of the various waveforms in the passband.

[coldcathode]

Ok,

It sounds like we are getting somewhere here.

The phase margin stuff is all "greek" to me so someone will have to decide on the proper margin?

I assume what we are talking about somehow involves prevention of oscillation due to the feedback signal versus the input signal "overlapping" at some point?

as far as the junction between the tube and chip,

Because the plate of the tube is at +100VDC Potential a bi-polar cap is needed to allow only the AC signal to pass. Otherwise By By chip!! The size of this cap is determined by the input IMPEDANCE of the chip. simple CR High Pass filter.

The GAIN of the tube is determined by the factors in the link RATZA posted. It is an EXCELLENT source of material. I started a few years ago in Tubes and used Aiken's stuff along with RCA RC-30 as the "BIBLE".

I now use calculators for the math. I can give you links later but it is imperative that you understand the calculations before using the "short way".

The capacitor that most affects the GAIN and SOUND of a common cathode amp is the Cathode Bypass cap Ck (C was already used for Capacitor so Cathode is Kathode)
Removing it allows negative feedback at the level of Rk, inserting it decreases the feedback 6db/octave above the F3 formed by the cap/resistor value.

So you see it is a compromise between GAIN and Frequency response. In addition the LARGE values needed to get a decent response (unless it is a mid-treble amp) require an Electrolytic cap, ie; not Audiophile quality.

Most TUBE designers looking for QUALITY and LINEAR response will AVOID Electrolytics at all cost and if more Gain is needed will insert another stage. For a Gheetar amp they are fine.

Daniel,

To clear up some other things.

1: We want to drive a higher impedance with a lower impedance not equal or the other way arround.

2: DO NOT USE a Headphone output. Line Level will ALWAYS be cleaner as the Headphone output is looking for a 32R or so load not 10 - 100K.

3: I have checked about 7 different sound cars ALL had output impedance of 10K or greater. Lowest Vout was about 200mV RMS @ 1Khz.

4: Gain is MULTIPLIED not ADDED. So a gain of 10 in the chip amp and 16 in the tube gives total gain of 160 X input RMS or in the case of the lowest card I tested 32V.
ie: .200 x 16 = 3.2VRMS 3.2x10 = 32VRMS

The above is EXACTLY WHY I need to know what the optimum design for the chip amp is.

5: I appreciate the defference being given to the tube amps "Dynamics" but in all honesty tubes are not much different than Chips overall. While a tube amp is a WHOLE LOT GENTLER when it comes to clipping and distortion, a WATT is a WATT.

For purposes of this project lets assume I am NOT building a woofer amp. I want a chip amp that can drive full range. We can "adapt" the design to match whatever range we want by playing with the input or place a filter between the input and a cathode follower buffer stage if need be. Lets make as "optimum" a chip amp we can. Maybe it is two different circuits for the chip amp. A low end optimized one and a fullrange/mid/treble optimized version. You guys are the chip gurus, let me know.