Hum when old BSR connected to new op-amp/speaker circuit

[indigo.girl]

Originally Posted by bikerhifinut

R1 and R5 are your input resistance setting resistors, R2 and R6 are the feedback and gain setting resistors.
Substitute R1 R2 R5 R6 for 470k resistors. Input impedance 470k, gain =1
or sub with 1Meg for a 1 meg input as i have a hunch thats a more appropriate input Z?

Excellent - I'm really getting there now! So its the ratio of resistors I use that determines the gain. I can use a pair of high value resistors to get a high Z input or a pair of low value resistors to get a low Z input, either way I get a gain of one if the resistors are the same value. Perfect

If I was to use a magnetic cart with low output Z and low output voltage I could use the same module but with low value resistors (to keep the input Z low thus matching the cart) but a combination of values that gives me high gain to boost the low output voltage from the cart.

[bikerhifinut]

Originally Posted by indigo.girl

Just looked up unity gain amplifier for use as an impedance buffer - it has high input impedance (ideal for the high Z cart) and low output impedance output (ideal for the amp) so I see how that works as a bridge to match the Z's . I suppose there's no reason why I shouldn't also set it to get some gain out of this too if the output voltage from my cart is a bit low?

Thats possible but when dealing with high input resistances with the way the op amp is configured, to get, say, a gain of 4x you need a 2.2meg resistor on the feedback. It'll work but i wouldnt go any higher as working at these resistances you can get a bit of noise. This is why the usual way is to use one stage just to impedance match and another for a bit of gain.

I doubt you'll need any gain here but I see no harm in putting a bit in and trimming the output with the output pot.

just be mindful, your power supply voltage needs to be AT LEAST twice the predicted output voltage at the op amp output to avoid clipping the signal.

I'd stick with unity gain meself.

[indigo.girl]

I just got this cheap stereo ceramic cart from china for £1.50 that I want to use in this project now my the original TC8M cart has broken (will obviously have to adapt the head shell to accomodate it).

http://www.ebay.co.uk/itm/1721297674...%3AMEBIDX%3AIT

I'm expecting the voltage output of this to be lower that the TC8M so perhaps I may need a little gain... but I'm presuming that this ceramic cart will have a high Z comparable to the TC8M. There's no data sheet with this cheap cart so I'm flying blind at the moment... any ideas what specs it might have?

[bikerhifinut]

output 200 -300mV tracking weight 5 gr
from another eBay seller.
It looks exactly like the cartridges on all those ion etc cheap turntables.

[indigo.girl]

You beat me to it - I've just found that info too....but no info on impedance.... should I just assume all ceramics/crystals are high impedence of about 1 Meg to be safe...

[bikerhifinut]

Originally Posted by indigo.girl

If I was to use a magnetic cart with low output Z and low output voltage I could use the same module but with low value resistors (to keep the input Z low thus matching the cart) but a combination of values that gives me high gain to boost the low output voltage from the cart.

Not really.
Yes you could boost the 5mV output of a magnetic to 500mV like this, but there's the issue of frequency correction. This is the RIAA stuff we talk about.
Far better to either get a cheap n cheerful magnetic step up stage or theres a velleman kit that does the job too. Ok for low budget stuff.
Oh and remember its horses for courses, a magnetic cartridge really needs a better arm to work. A starting standard would be something like the Good Ol' Garrard SP25 or one of its competitors.

A.

[bikerhifinut]

Originally Posted by indigo.girl

You beat me to it - I've just found that info too....but no info on impedance.... should I just assume all ceramics/crystals are high impedence of about 1 Meg to be safe...

Yes, because they are.

[Clydeuk]

Those cheap cartridges I've found work very well, and can give good quality sound depending on what they are hooked up to.

I wouldn't think it was possible to damage a cartridge because of impedence mismatching - you could short the two cartridge connections together and it wouldn't damage it. I suggest you try your TC8M in another player to check if it still works - it would be unusual for it to have been working and suddenly fail.

[indigo.girl]

Just given my three carts a quick test in my amp set up. The TC8H works nicely as it did yesterday. Inexplicably the TC8M also works today but at a lower max volume than the TC8H (as expected from the medium output from the M wrt H). My new stereo cart from china is great. It temporarily wedges snuggly into the metal case of the turnover cart holder . I've just wired up one of the channels and I get a good volume from it through my little amp and speakers. This is all without the impedance matching we've been talking about. What kind of improvement would I expect to see if I now matched the impedance with the unity gain pre-amp? Is this a sound quality improvement or a volume improvement?

NB: the volume is ok to listen to but the quality is distorted and there's definitely room for improvement (that I'm hoping the impedance matching will do this).

[indigo.girl]

I've just swapped my tiny 6.5cm speakers that came with the kit with a big 9" speaker from inside a Philips 654A radiogram. The volume and sound quality is much better with this speaker.

[Clydeuk]

Impedance matching would probably get rid of the distortion. Also paralleling the 2 channels on the cartridge would improve the sound as at the moment you're only hearing one channel of the stereo pair, alternatively since your little amp is stereo you could remove your bridge on the input and wire in both channels.

I've managed to wedge one of those carts into an X3M bracket before, never tried it with a TC8 as I've not had a player with one of those for years.

Another advantage of your new cartridge is that it won't damage stereo records the way a TC8 would.

[bikerhifinut]

Its difficult to say.
For sure matching the impedance is a good thing.
I have a feeling that your little amp and speakers just ain't that good and you might not be getting the best out of things.
You could try a pair of headphones and that would indicate if the amp itself is ok and its the speakers that are the issue. They'll sound pretty tinny anyway whatever you feed into them.
The main thing about impedance matching so to speak, is that you don't lose the low frequency parts of a signal. This is very simplified. You also lose a bit of the signal but I doubt its that much in this instance.

A.

[bikerhifinut]

Ah it would seem you've tried a better quality speaker.
Making progress then

[peter_sol]

Are you using the 430 K still with your tests?

[indigo.girl]

Originally Posted by peter_sol

Are you using the 430 K still with your tests?

Yes Peter. The 430k is still wired in so actually the impedance is matched to the cart. I should remove and compare the sound....

[indigo.girl]

Originally Posted by bikerhifinut

Ah it would seem you've tried a better quality speaker.
Making progress then

What would be the optimal speakers to use with this 1W channel output amp but staying small in size like they had in the old dansettes. I'm thinking of mounting it all in a case. This 9 incher is way too big!

[bikerhifinut]

Something salvaged from a scrap radio.

The biggest that you can fit easily in your chosen cabinet.

Why not put everything back in the original gram cabinet and use the speaker in that?

A.

[Edward Huggins]

I have already suggested 5" and 6.5" diameter units. Alternatively 7" x 4"ellipticals. Edward

[peter_sol]

As per your question in post 89
Possibly nothing to gain and a waste of time and effort.
What is the volume like with the new cart and better speaker?
You need the speaker on its baffle or in its case to judge or mount the small speakers on a baffle you can't just can't leave them out in the open. For a test you could get a piece of cardboard an cut holes and mount them on that.

[Edward Huggins]

Completely not sure where this thread is going....is this a learning experiment or is indigogirl trying to make some kind of record player out of an old BSR autochanger with a dubious mono cartridge, a low-spec stereo IC and 2 very small speakers? In the meantime, and somewhere in the background lurks, an old Sobell mono radiogram cabinet. All rather surreal. I mean what is to be the final desired outcome? Edward