Air Vane RIAA Tuning Question

[soulmerchant]

Hi all,

I searched this topic, and only found a small reference to it so far.

In a previous post I shared the schematic of a very nice grant phono amplifier (ca. 1986) that I recently got for free and refurbished.

The schematic:
http://www.diyaudio.com/forums/attachment.php?&postid=1037705

I've made a few small changes to it thanks to some kind advice from this board.

It now sounds superb, and with a nice step up transformer like a stevens and billington tx-103 it frankly blows away the stock version of the ear834p.

I like it so much that I have started to build a mono version (just for mono playback). This is where it gets a bit silly though...

Many of old mono records were not mastered to the same standardised RIAA curve. Some were done to completely other curves, and 78's are a completely other story...

Ok, you're thinking that I'm going high-end bonkers, but I want to use air-vanes to tune the RIAA curve in the global feedback circuit.

The question is this:

What kind of air-vanes sound good? I'm currently looking at Hammarlund air variable silver plated ceramic. Kind of pricy, so does it really matter?

And should I just do the 270p and 27p values in the feedback loop, or should I consider the 8.2p (on valve 2b)....

I've been looking at this site http://www.surplussales.com/Variables/AirVariables/AirVar2.html
any other good choices?

Perhaps I should be asking at a ham radio forum


Edit : spelling

 

[tvi]

Rommy the cat uses these AVC150/BC14400 .

Or get real tweaky and remove the final dielectric with Vacuum variable caps , not cheap though.

Hope this helps

Regards
James

 

[soulmerchant]

Thanks James,

I don't know if I can trust someone called "romy the cat" , but the site you listed looks great.

I also received some other fine advice - the 1000p and the 270p values should be tuned - and some resistors as well.

I'm also not shure what sort of ranges I should be looking at. If necessary I will just experiment with different values until I find a good working range.

Ian

edit: my spelling is poor after speaking german for so long now...

 

[jackinnj]

a variable cap with 270pF max -- you can probably get a tuning capacitor from a tube radio at the salvation store for next to nothing including the radio-- these were usually 365pf (we used to call them 365mmF )

 

[jackinnj]

Just a couple of tweeks -- a lot depends upon the SPICE model you use for the ECC83's -- but it looks pretty darned good -- shown is deviation from RIAA curve :

 

[soulmerchant]

Thank you very much jackinnj. That is solid gold. I will definitely try out those values. What simulation software are you using?

I've been using el cheepo chinese 12ax7a's in it. No fancy paper in oil caps either - just cheap polyester - but the sound is there. Really tight bass, fast delicate highs and very musical. My friends ear834 sounds somewhat muddy in comparison. The small value caps are polystyrene - hard to source these days...

 

[EC8010]

The small value caps are polystyrene - hard to source these days...

Try LCR Components in Wales. Farnell distribute them.

 

[jackinnj]

Thank you very much jackinnj. That is solid gold. I will definitely try out those values. What simulation software are you using?

I've been using el cheepo chinese 12ax7a's in it. No fancy paper in oil caps either - just cheap polyester - but the sound is there. Really tight bass, fast delicate highs and very musical. My friends ear834 sounds somewhat muddy in comparison. The small value caps are polystyrene - hard to source these days...

you can use silver mica for the small value caps -- not too difficult to find

i just use a regular simulation program Multisim -- to make the reverse RIAA network you can use a LaPlace transform -- Walt Jung describes it in his book "Op Amp Applications Handbook" -- or you can use a transfer function block if your sim program has one.

 

[SY]

One caution: air vane caps tend to be very microphonic. As appealing as air-as-dielectric might be, you might do better with a Teflon trimmer.

 

[soulmerchant]

One caution: air vane caps tend to be very microphonic.

Yes, but I think it is worth a try. I will try to stay away from anything long shafted - i hope that will reduce rf signal sensitivity.

I want to be able to adjust on the fly - and probably often, so a telflon trimmer may not hold up over time as well. And the air vanes are supposed to sound really good....

I'm thinking of just doing the 270pF (range up to about 400pf) and also trying different values of the 12M and 270K resistors for starters. I'll try to keep you updated...

 

[jackinnj]

One caution: air vane caps tend to be very microphonic. As appealing as air-as-dielectric might be, you might do better with a Teflon trimmer.

I recall having a Heathkit VF-1 VFO back in the 1960's -- all you had to do was breathe heavily and the frequency would shift.

In SoulMerchant's application, however, a few PF one way or another isn't going to matter much, as it isn't (hopefully) an oscillator. The capacitance of a variable cap will also change with the relative humidity. Best bet is to hook up the preamp to an Inverse RIAA network, then adjust the 270 pf cap (using a 365pF air-variable) for best response -- the 12 meg resistor will also need adjusting. When you have determined the optimal value swap out the air-variable and substitute in polystyrenes or mica for the exact value. For very small values of capacitance you can twist together a pair of wires -- called a "trick" capacitor.

 

[SY]

I have in front of me my QSL card from 1968 with... a Heathkit VF-1 visible in the photo. It wasn't until I finally got a Collins 32S-1 that I ever experienced stable operating frequency.

Completely agree about the use of a trimmer to determine the right value, I just wouldn't want to leave it in. Microphony means generation of a voltage at the resonant frequency. A small voltage, I grant you (I haven't calculated an order-of-magnitude), but right at the most sensitive point for human hearing and in an environment where there's lots of vibration in the air to excite it.

 

[radtech]

I agree, use a variable to determine the value, then replace with fixed. Judging from the 365pF caps I've used in radio stuff I can imagine they might be very sensitive to vibration right in the audio range, some of them made a nice little tone if you ran your finger over the plates.