caruso preamp pcb boards for sale..

[flozki]

hello my contribution for a summer holiday project....

i have first batch caruso preamp boards ready for sale.
the pre manufactured pcb with soldering mask and silkscreen and gold plated contacts.

for sale:
SFR 40.- (used to be $40) plus shipping. (approx 5.-)
sorry i can not give prices in $ anymore. free falling.

this board contains:
-riaa encoding (switchable 75us time constant)
-feedback preamp mixer, and
-feedback monitoring section.
all parts can be bought at mouser or any electronic shop. i used through hole parts only.
the preamp works for caruso preamp, sc99. neumann es59,sx68,sx74, ortofon heads. for stereo you need 2 boards.

there a few parts which are specific to adapt to the head and prefered cutting premp for best performance. but in general it works for any cutterhead. you can also use for non feedback type heads. just dont use the feedback.

additionally you need a good and nice +/- 15V powersupply. (transformer, rectifier a few caps and 7815,7915) and a case.
depending on quality and parts the whole project for a stereo version
will cost you $200-400 in total for all puls 1-2 days of time.

schematics, parts list will come with the board. i will also post the schematics. as soon as i built it up myself. including pictures and infos...

and if you ever buy (yes its still my fault...) i deduct the sfr 40.- on your caruso buy.

f.

i'm off for 10 days. so boards will sent out not before august 12th

[bancho]

GEIL!

Is it possible to use it with Grampian type D also? I mean the feedback circuit...

[Nickou]

ok , we are the 12 ...

can t wait more ...

[Nickou]

HI Flo ,


Tomorow monday

sales are begening ?

[flozki]

yes. all order please pm
or even better
to my email adress.

i have to figure out shipping. then paypal.

i solder together tomorrow the first board...for tests.
f.

[Nickou]

the boards arrived this morning ...

[flozki]

for the ones who have the boards already.

i put together a site, blog for the amp.

but in short :

changes and part values for preampboard v10

C403,R401 not necessary.
bridge under the riaa-switch pin
add an output resistor (47-100) between output of opamp and 6.3mm jack socket.

opamps.
i tested with lowcost tl072.opa2134 is great. ne5532 should work perfect and sounds great for its price.

-r309,c316 is the input low pass. set that betwenn 22kHz (hobby) and 50 kHz for professional use.

-C400,402 can be somewhere between 10uF and 47uF to be tested
-R606 head specific (gain for fb-preamp)
-R405,C403 head specific (PI amount) 26 for neumann and sc99. if you are not sure or use a non feedback type.bridge r405 , leave out c403
-C405,R407 is the D-amount. if you are not sure or use a non feedback type.bridge r407 , leave out c405


happy bricolage....

[d]

hey

Flo, it would be great to get the schematics for this preamp

[Nickou]

I just finished to put the parts on the boards ,

everything seems to be ok ,

I am thinking to make some test with a sx74 a hifi amplifier I have
and later with the sal

any recommendations before making any tests ?

[flozki]

hello yann. and d. for you i guess it is especially interesting...hehe.

great looking forward for your tests...hope you did all the changes as described in my last post here.
definitely check it before do some tests with the head.
feed in 1 khz and check if riaa works. and if all signals are clean.

i just did some testcuts today with caruso preamp and a ortofon ds522 which i had to fix and the customer picked it up and we tested it.

so i just used the preamp with the standard values only (shitty) tl082 and ssm2017 in it... except the feedback gain.
beginning it was on 470 =27db gain but got only 4 dbs of feedback
then lowered to 330ohm (31.2db gain) and then got around 6.8db of feedback at 5 kHz.
i still have not enough experience what is the best value at 5 khz.
i used my bruel &kjaer labor amp and NO rc network. so the whole regulation circuit was not perfect.

with around 5db's at 5khz the sound was already great. a little limited in the highs because of missing rc network and current limitet of 1.8A on the amps..

so no big problems should occur... just make sure you got rc network (2u2 in parallel with 4.7 ohm ) and fuses 0.5 fast for first tests in series... and make sure riaa is off for feedback adjustments..
the hifi amp could give some troubles..but normally not..

i think the preamp sounds great and is very easy to handle..

let me know.

[Nickou]

hello yann. and d. for you i guess it is especially interesting...hehe.
.

yes , it is .... !

and make sure riaa is off for feedback adjustments..

I will , but why is it important ? security ?
many thanks for this very happy bricolage !

[opcode66]

If you have RIAA on and you feed a 5K signal to the head it will be greatly amplified due to RIAA and you risk blowing your head. At 5K RIAA raises the signal 8.2 db. You have to do the same thing when calibrating the SAL or VG racks.

[Nickou]

I was not asking what is the riaa curve
but why it is needed , if it is only for security reason, or also for something else.

[opcode66]

and make sure riaa is off for feedback adjustments..

I will , but why is it important ? security ?
many thanks for this very happy bricolage !

If you have RIAA on and you feed a 5K signal to the head it will be greatly amplified due to RIAA and you risk blowing your head. At 5K RIAA raises the signal 8.2 db. You have to do the same thing when calibrating the SAL or VG racks.

I was not explaining what the curve was. I was saying that you turn it off when calibrating an amp circuit, specifically when adjusting the feedback via a 5khz tone. That is why Flo said to turn the RIAA off (bypass it).

The following is from Neumann Paper# 21669-920-002.01 Installation and Calibrating Instruction VG74/SX 74

4.3. Calibation of the Cutterhead SX 74
Setting feedback:
4.3.1. Switch the RIAA OUT/In switch in the SAB 74a to the OUT position.
4.3.2. Set the MONITOR SELECTOR to the FB position.
4.3.3. Turn the stylus heat potentiometer on the lathe down to zero (to the left).
4.3.4. Lower the cutterhead to the right of the turntable by leaving the lead screw engaging lever in its vertical postion and pushing start.
4.3.5. Turn the FEEDBACK potentionmeter all the way to the left and push the OPER button on the SEL 74.
4.3.6. Feed 5 kHz with a level which indicates 0 VU = +4 dBm on the MON. PREAMPLIFIER OUTPUT. (The MON. button pushed on the SP).
4.3.7. Turn the FEEDBACK control to the right until the VU-METER level on the MON. PREAMPLIFIER OUTPUT drops by approx. 9dB (see check-out report SX 74).

There is a bit more, but that is the procedure to adjust feedback. As you see in the first step the RIAA curve is turned off. Why? Because otherwise if left ON you take a 5kHz signal at +4 dBm and raise it an additional 8.2 db at a time when you have the feedback set to zero. That would likely cause problems. Like blowing your cutterhead for example. That is what I was saying. It is bypassed when calibrating at the point when you are adjusting the feedback.

[flozki]

exactly
thanks for short summary opcode
just much safer to adjust with riaa off..but you can also do with riaa but i woud not...

especially with a new system you dont know....and which might have not optimal parameters...or polarity is not 100%

by the way if you have a dummy head...make tests wth the dummy before. mybe you can apply only a few dbs of feedback. but then you know all works perfect..

let me know how it works hehe.

[Nickou]

when you adjust the feedback level, all the riia encoding is before the drive level , mixer etc ...

Even if there is a feedbacxk loop ,it won t change nothing to have riaa on or off .


It is right than with the sal , when you switch off the riaa , you also limit the power of the amplifiers, and , only that is a good reason to switch off the riaa ...
But I am going to make those test with a Studer a68 amplifier, not with the sal, so with RIAA on or off , the amplifiers will give the same power and the security level of the head will be the same

let me know if I am missing something...


ok , for the tests , RIAA will be , as ever , off...

exactly

by the way if you have a dummy head...make tests wth the dummy before. mybe you can apply only a few dbs of feedback. but then you know all works perfect..

let me know how it works hehe.

don t worry ...I ll put a camera focused on the head, if something explode or burn , it will be documented

[opcode66]

Of course everyone is free to make their own decisions with respect to how they use their gear or what they do with it.

However, you've got the inventor Flo telling you the same thing as the engineers from Neumann. I'm inclined to follow their advice and guidance.

When you say "RIAA will be, as ever, off..." are you seriously saying you cut without the RIAA curve on? That it is always off? How do you compensate for the de-emphasis inherent in every playback system?

To my knowledge swtiching the RIAA off on a SAL rack has no bearing on the power of the amplifier. It is simply a switch that bypasses the RIAA circuitry. I can double check my schematics. Anyone else want to chime in on this?

[dubcutter89]

when you adjust the feedback level, all the riia encoding is before the drive level , mixer etc ...

Even if there is a feedbacxk loop ,it won t change nothing to have riaa on or off .

that's right

At 5K RIAA raises the signal 8.2 db

+8.2dB signal means 4 times more power to the head... you can do if you want to, but i would not. -> RIAA off

Maybe this helps...

Lukas

[dubcutter89]

@flo

did you modify the circuit for the ds522?

my measurements told me that the head has a roll-off at approx. 200Hz
(even with fb) so the RIAA low cut has to be modified...

where do you put the rc-network?

in parallel/series with the drive coil? for boosting the highs?
i've been looking at the go511 schematic for reconstructing the fb signal path,
but i'm not finished yet...

Do you have your custom fb-coils now? would be interested...

Lukas

[flozki]

@flo

did you modify the circuit for the ds522?
my measurements told me that the head has a roll-off at approx. 200Hz
(even with fb) so the RIAA low cut has to be modified...

i used pi amout similar to neumann head. so thats a boost of 26 dbs in the lows (within the regulation loop)
bass response was perfect already with 4-5 db's of feedback @5k.
almost flat within 0.5dbs

rc network goes in series before the head. with a regular amp. it hels to provide a constant load for the amp otherwise impedance rise...
all cutting amps do that in some kind....

i havent lounched the feedback coils. because noone is willing to pay something... so as long as i dont need one for myself i wont do it...
but i can just order...hehe.but be prepared to spend 300-400 bucks and then you have perfect feedback coils.
if there are 4-5 people we can lower it

f.