PASS principles super tweeter amplifier

[rickmcinnis]

After seeing how the simple capacitor I use for the crossover of a FOSTEX T500 affects the response of my JBL2441/Truextent I am thinking I need a dedicated amplifier for these. Maybe I could figure out how values to keep it from creating a rising response but I would rather just power the T500s separately and not touch my crossover for the 2441s.

Cannot afford an SIT1 for just the tweeters - I could not afford the SIT1 to begin with but I had to have it.

I would like a single stage design since I am lazy. I do not have a problem with caps on the inputs and outputs - would not have a problem with transformers, either.

Crossing over at 9 to 10KHz so very little power is required. Of course, a first order slope will be used so it needs to be GOOD sounding since it will influence frequencies below for an octave or so.

I was thinking about a spud amp using that Russian tube that Romy uses but I thought I should stick with the PASS signature since the range above 100 hz is driven by First Watt amplifiers and a tube could lend discontinuity to the sound.

Suggestions will be greatly appreciated.

 

[Zen Mod]

ACA

size output cap to your needs

 

[rickmcinnis]

I am embarrassed to have to admit I do not know which circuit is the ACA!

Please point me in the right direction, ZM.

THANKS,

 

[rickmcinnis]

AMP CAMP AMP - aha! If only I would search before asking ...

Thanks, again.

a minute of two later:

I have most of those parts on hand. Ordered the PCBs from THE STORE.

Not too sure about using switchers but then I have phobias.

What would you think about lowering the amount of feedback to get some more gain?

I have this unbearable desire to do SOMETHING different with it. Likely I will leave it alone - If I need gain I will put an autoformer in front of it.

Will it be happy with just a 1uF output cap? Sure hope so ...

 

[Zen Mod]

ya can call it Poor Man's SIT amp

 

[vdi_nenna]

Rick,

The ACA manages to sound more powerful than the 6w suggests. Not sure how it does it.
I built it with a linear supply and it sounds fine to me. Never heard it with laptop supplies, so I can't comment on the sound with bricks.

Vince

 

[wdecho]

Rick,

The ACA manages to sound more powerful than the 6w suggests. Not sure how it does it.
I built it with a linear supply and it sounds fine to me. Never heard it with laptop supplies, so I can't comment on the sound with bricks.

Vince

Same for me exactly. I was shocked at the sound from these little amps. Mine is stereo in the same case, linear supply much as all the FW stereo amps.

 

[rickmcinnis]

I will not be needing much power but would like to think it will work well with the FOSTEX.

Not sure how demanding they are since I have only heard them with the SIT1.

Will know in about two weeks.

THANKS for all advice and counsel.

 

[rickmcinnis]

ya can call it Poor Man's SIT amp

Well, if we replace the current source with resistors could we come closer to a poor man's SIT1?

Or is this the place to try choke loading the MOSFET?

Any thoughts?

Just found ZM's SHADE version. ZM, should I try this?

 

[Zen Mod]

feel free to try everything you want

ACA - it isn't simple CCS above , that's sort of Mu Follower

but , who knows , maybe you'll prefer resistors instead

I can't remember that I made Shade version ....

 

[Nelson Pass]

I know people who love resistive current sources. Actually, the SIT-1
uses that. Halves the efficiency though...

 

[rickmcinnis]

I seem to recall a well known amplifier designer/theoretician who likes them.

Efficiency means nothing in this application. This amp is wildly powerful for driving a tweeter above 8kHz (I would think).

Would Ing. Pass be willing to tell this soldering tinkerer how to proceed? If you get me started I think I can figure it out ...

That last sentence is probably wildly optimistic ...

Take care,

 

[Zen Mod]

not Ing. Pass himself , but eternal adoptee ......

if you're keen to do that ..... make ACA in original iteration , then listen

after that you can replace upper half with few power resistors

will post schematics soon

 

[Zen Mod]

after that you can replace upper half with few power resistors

............ then listen .......... leave whichever iteration you like most


again - output cap goes in size and flavor of your liking

 

[mitchba]

I use ACA's to drive compression drivers in a multi-way system using digital XO.
Love the sound! So sweet, yet detailed, but never harsh. These are stock ACA's using the switchers, no noise or buzz at all. You can't go wrong.

 

[rickmcinnis]

ZM,

If you did not exist NP would have had to invent/create you!

In other words, THANKS VERY MUCH for the help and even more for your existence.

Take care,

 

[Zen Mod]

naah ........

5R625 - not exactly value carved in stone

however - you can find them easily at ,say, mouser



chassis mounted (usually goldish) , 25W resistors

two of 1R8 and one of 2R (or 2R2) , all series connected , will result in nice 75W resistor group

you can get them for under 20 bucks (for all 6) ....... shipping excluded

http://eu.mouser.com/Passive-Compon...s/_/N-7h7zf?P=1z0wncuZ1z0x737Z1z0x703Z1z0wn9m

 

[rickmcinnis]

Thanks, ZM.

Michael Percy has some non-inductive goldish resistors - does non-inductive mean anything in this application?

Sorry to keep pestering you about the feedback - do you think there could be any value in increasing the value of R12?

 

[Zen Mod]

I'm not overly bothered with "non-inductive" resistors , finding usual Dale-ish goldies enough non-inductive for my liking

feedback - same as I wrote for trying active vs. resistive loading - try original ACA , then change one thing at a time

why should you want changing stage gain , in first place , without any defined starting point ?

 

[rickmcinnis]

To satisfy my curiosity - I read that some have recommended using high voltage B+ - is this simply for more power or is there a sonic benefit to doing this?

The reason I keep going on about the feedback is to adjust level if that is needed; there is the 4 dB difference between ACA and SIT1.

I will be using a crossover arrangement similar to what I am using for the JBL/Truextent on the SIT1 - both input and output caps sized for the crossover point but with the addition of an inductor for a .5 Q slope.

I have found with the JBL that an Lpad/damper really makes an audible and measurable difference and for that i will likely need the extra gain. If you think the distortion from low feedback would spoil the whole thing I will dismiss it now.

Do you think a 2200pF input cap could cause problems?

THANKS!