Line-level attenuator circuit for recorder

[Steve A.]

I've been trying to do 24 bit vinyl rips and the signal from the phono preamp is a little too hot for my M-Audio MicroTrack 2496. I'd like to cut the signal level in half, while maintaining proper impedance and frequency response.

I was thinking of wiring up a L-circuit. Something like a 1k resistor in series from the phono preamp, and then with a 1k resistor to ground on the output going to the recorder. Of course, this would be doubled for stereo.

Any ideas how this will work? Should I change the values...?

Thanks!

Steve A.

[Enzo]

The signal from the preamp is not balanced is it? So why wouldn't a plain old vanilla volume control on the output work? Two 1k resistors in series from output to ground and take the signal from the center is just a volume control at halfway. I don't think the Hi-Z is all that picky. personally I'd use 10k or higher instead of 1k.

[Steve A.]

This is line level and not Hi-Z- I just wired up an adaptor cable with 1k resistors, wired up as a voltage divider, from the point of view of the microrecorder. I'll let you know how it works.

Thanks!

Steve Ahola

[Enzo]

Sorry, never seen a low Z phono preamp out. Most non-balanced line level outputs I see are Hi-Z.

[Steve A.]

Sorry, never seen a low Z phono preamp out. Most non-balanced line level outputs I see are Hi-Z.

Okay, well I think of low level guitar pickups being Hi-Z and any kind of line level signal being low Z. My bad! I just looked up the specs and see that the output impedance is 1K ohms. (Then again my strat has a DC resistance of ~3k with two pickups in parallel- would that be Hi-Z or low Z?)

With that in mind what values would you use?

Thanks!

Steve Ahola

[Enzo]

Grab a 250k pot or something, I don't see it as critical, you just need a voltage divider. That may not be optimal, but it would demonstrate feasability.

Line levels can be Hi-Z or Low-Z. LowZ signals are most often found in balanced lines.

[Steve A.]

Grab a 250k pot or something, I don't see it as critical, you just need a voltage divider. That may not be optimal, but it would demonstrate feasability.

Line levels can be Hi-Z or Low-Z. LowZ signals are most often found in balanced lines.

Well, I didn't want to have to put this in a box, so I think I'll skip the pots. I wired up a 1/4" stereo cable with the 1k resistors on the metal jacks. I guess maybe 10k resistors would have been better, judging from your earlier post- but the 1k resistors seemed to have worked okay.

Isn't there a rule of thumb that the impedance of the input should be 10X the impedance of the output from the preceding stage? Not that your thumbs will be amputated if you violate that rule...

Thanks!

Steve Ahola

[Enzo]

Just like there are rules that say 6V6s should run with 100v less on the plates than Leo Fender put there.

I was thinking a pot and some clip leads to test the theory and tweak your values, rather than build a boxing, but if it works, it works, and that is the bottom line.

[Steve A.]

New question: How to determine dB attenuation?

I need to make up another cable for a friend and we would like it to have 6dB attenuation. With a 10k resistor going to ground, what value series resistor would be used?

Another way to put this: what ratio reduction in voltage would correspond to a 6dB attenuation in signal level?

Thanks!

Steve Ahola

P.S. I just checked this site:

http://www.sengpielaudio.com/dB-chart.htm

... and it looks like reducing the voltage in half will attenuate the audio signal by 6dB. In which case the 10k series resistor and 10k resistor to ground would cut the voltage in half.

P.P.S. On a slightly different subject, I was wondering if anyone has any experience converting an MM (moving magnet) phono cartridge preamp for use with an MC (moving coil) cartridge? In particular the BBE FJB-200X seems to have plenty of gain to handle the 0.5mV output of my new MC cartridge (a Benx MC20E2 L) but the impedance match isn't optimal. The input impedance of the preamp is 47k, and the cartridge will "work" with anything up to 47k, but would be "happier" seeing an impedance of around 1k. I guess if I put a 1k resistor on the input jack of the preamp input it would bring the impedance down to around 1k, but it would also decrease the gain significantly.

This is a moot point because I do have a Denon AU 300C step-up transformer coming in, which is used to connect an MC cartridge to an MM input stage. (Many people recommend the transformers instead of additional gain stages because the S/N ratio is better.) The Denon AU300C comes with a selection of optional loading resistors- and instructions in Japanese! Now if I can just brush up on my Japanese enough to figure out the instructions... NOT!

[Bruce / Mission Amps]

What you also have to be careful with is that the total of 20K across your source signal will not load that side or the 10K load the "receiving" side down.
That is the matching part for proper signal transfer minus the 6dB drop.

[dai h.]

hey Steve, there's a project shown here to step up the voltage from an MM cartrige using a cheap 100k to 1k transformer (backwards) :

http://www2.famille.ne.jp/~teddy/pre/pre3e.htm

some of the apparent points/keys seem to be:

the low output impedance of the cartrige(helps low freq. response from transformer), the secondary termination R(gets rid of high freq. peak but if too low reduces output voltage so 100k was about right all things considered), and added electrostatic(copper tape) to the core (also the phono EQ the transformer is connected to has input Z of 47k(usually)

oh and if you want to try scanning the instructions I can try reading it but don't know if I can come with technical answers

[Steve A.]

hey Steve, there's a project shown here to step up the voltage from an MM cartrige using a cheap 100k to 1k transformer (backwards) :

http://www2.famille.ne.jp/~teddy/pre/pre3e.htm

some of the apparent points/keys seem to be:

the low output impedance of the cartrige(helps low freq. response from transformer), the secondary termination R(gets rid of high freq. peak but if too low reduces output voltage so 100k was about right all things considered), and added electrostatic(copper tape) to the core (also the phono EQ the transformer is connected to has input Z of 47k(usually)

oh and if you want to try scanning the instructions I can try reading it but don't know if I can come with technical answers

Hey, thanks for the link. That project looks really great- at least if I ever learn how to read Japanese! The audio transformers look like the ones I've been getting from Mouser- the ones I've been using as inductors for my Craig Anderton/Dan Torres mid controls for guitar. So that would be an audio transformer with an input impedance of 100K and an output impedance of 1k? I found one at Mouser with an an input of 50k and an output of 1k ($1.91 each):

42TM117-RC

I received the Denon AU300LC step-up transformer and it really kicks up the output of the MC cartridge- so much so that with the BBE FJB-200X phono preamp it overloads the input to my M-Audio MicroTrack II. But it sure works great with the Bellari VP-129 tube phono preamp...

Steve Ahola

[dai h.]

hey Steve, yes it's one of those dinky, inexpensive types. The one in the article is an "input transformer" about USD$6, 100k to 1k (DCR1.27k, 45ohms), ratio 10.1:1. Here's a similar one (in the 'States) for example:

http://www.oselectronics.com/ose_p119.htm

100k to 1k (DCR2k, 70ohms), $5.95.