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

[peter_sol]

Most basic tone controls simply reduce the treble content of the audio but with a decent speaker (Try the one in your gram) may not be needed. Do you have any pots and capacitors around to try?

[Edward Huggins]

Do note that that very rare and valuable BSR TC8H high-output cartridge you plan to use is liable to overload the input stage of your IC amplifier. It gives around 1.0V output. Also those little speakers are not going to give very good audio as their diameter is so small. Edward

[Herald1360]

Found the datasheet for the chip- its input resistance is 100K so I would think that increasing the pot(s) to 50K would be reasonable and would give a little more gain.

If you're feeling adventurous, you could change the circuit to the bridge output version in the data which would only need a single volume pot and connect the speakers in series between the two outputs as shown.

[Clydeuk]

I actually find it incredible that any TC8H cartridge still works. I've never encountered one that worked, even back in the 80's. The TC8M seemed to last longer from my own experiences.

[indigo.girl]

Originally Posted by Clydeuk

Just a thought on your capacitor connected to the "battery" terminals. If this is designed so that a battery could be connected while still allowing operation from the adaptor, the power socket may be switched to disconnect the battery when the adaptor plug is inserted. So your capacitor in place of the battery may be disconnected. A meter check would tell you if this is the case.

Good point. It does say in the notes that come along with the IC amp kit that "if a power supply is connected to the DC socket the battery is automatically disconnected" So I guess my capacitor hasn't been doing any thing. There's not much difference with or without the capacitor now I'm using the TC8H cart so I'll just remove it for now.

[indigo.girl]

Originally Posted by Edward Huggins

Do note that that very rare and valuable BSR TC8H high-output cartridge you plan to use is liable to overload the input stage of your IC amplifier. It gives around 1.0V output. Also those little speakers are not going to give very good audio as their diameter is so small. Edward

I'm wondering then if thats why the TC8H seems to work ok where as the TC8M does not. The latter is not broken simply it has much less output so the audio is extremely low. What is the voltage output from the TC8M compared to the TC8H. I don't really want to use the TC8H in this - it was simply to test the system. Should I persevere with the TC8M and see if I can increase the gain of the amp system?

Interestingly, I'm not sure that this TC8M was the original cart in this deck because the pick up arm had 4 wires inside (with two unused) as if it once had a stereo cart (although surely not in this old 1950's radiogram)....

[peter_sol]

They were all "wired" for stereo even if a mono cartridge was fitted.

[ms660]

Post#35?

Lawrence.

[bikerhifinut]

Originally Posted by indigo.girl

I'm wondering then if thats why the TC8H seems to work ok where as the TC8M does not. The latter is not broken simply it has much less output so the audio is extremely low. What is the voltage output from the TC8M compared to the TC8H. I don't really want to use the TC8H in this - it was simply to test the system. Should I persevere with the TC8M and see if I can increase the gain of the amp system?

I aint 100% sure, but I have a feeling the reason why its so quiet is because you've made a rather large attenuator by wiring a large value resistor in series with the input. Its still a potential divider you see.
draw the circuit out and you will see where I'm coming from.
Unfortunately, as has been pointed out, the amplifier chip has a maximum input Z of 100k which is plenty for any modern source but with a piezo cartridge you ideally would need around 1meg.
There is a solution and its not difficult or costly.

A.

[indigo.girl]

This looks great Andy - and at £4.99 its a bargain. I paid £12 for my kit. It says new old stock - so has this module design been around a while? Its pre-op amp I'm presuming, I can't see from the photo...

I'll soon be getting a stereo cart that I can use to replace the TC8M (I have one on order from china for a couple of quid). Do you know the impedance of the this module so I can match it with the cart when it arrives? And can I use my original speakers with this module or should I give up on these little things too?

Where's a good place to source modules like these ones? I found mine at Maplins but they don't have much to choose from. Where's a good stockist that will help a newbie?

[bikerhifinut]

Originally Posted by pwdrive

Post#35?

And posts#8 and #9 Lawrence.

A.

[ms660]

Yes, it's the old impedance problem again.

Lawrence.

[indigo.girl]

Originally Posted by indigo.girl

This looks great Andy - and at £4.99 its a bargain.

Oh, your comment has disappeared Andy - the one about the other module from Sheffield... this is what my comment was replying to

[bikerhifinut]

Hi Indigo girl, i apologise as i deleted the post more or less straight away as possibly not relevant to the thread.
Its from bardwells, I'll repost the link here.
I'd suggest you'll still need to get an impedance buffer for use with the cartridge but thats fairly easy and I can give you a socket and an op amp that will make one, or you just adapt one of the cheap opamp pre's from maplins.
A few of the members of this forum have built amps using this module with a lot of success so you'll get plenty of advice based on experience if you do buy one.
http://www.bardwells.co.uk/product/s...s-per-channel/

Andy.

[indigo.girl]

Originally Posted by pwdrive

Yes, it's the old impedance problem again.

Ah, so are you and Andy saying the 430K resistor in series with the input channel is causing me problems? I'm getting a little confused as I thought the 430K I've added here increases the impedance of the IC circuit from 20K to about 500K hence matching the requirements of this Piezo cart? Can I wire it differently to increase the impedance without causing the 'attenutor' problem?

[bikerhifinut]

One for the transformer experts.
Is it possible or even practical to use a Step down transformer with a piezo cartridge.
e.g. cartridge with Z=500k ohm into a 10:1 transformer gives an output Z=50k? and a tenth of the output voltage which may or may not be a bad thing?

If this is OT on this thread mods please do as you see fit.

A.

[bikerhifinut]

Not really Indigo Girl.
Whatever you do, you get either an impedance mismatch to the cartridge or excessive attenuation.
The only really practical way as I understand it, is to have a buffer stage in between. This is seen commonly in amplifiers, in valve amps you'll see perhaps a cathode follower to provide an impedance match and in transistor amps it would be what they call an emitter follower.
Sometimes transformers are used too. In deed that is exactly what a loudspeaker output transformer on a valve amplifier does.
Nowadays we have these wonderful little devices called op amps and they are very versatile. So good that they are found in the highest of fi and pro studio kit.
The good news is that they are cheap and for the cost of one of these (pence in many cases) and half a dozen small resistors and capacitors you can make a unity gain impedance matching buffer, or a high gain quality amplifier and everything in between.
You may guess that I'm a fan of op amps and you'd be right.

The consolation here is that with that little amp and those miniscule speakers you'd probably not notice any fall off in the bass frequencies due to the mismatch, so I'd go back to the drawing board and simply reduce the value of the series resistor until you get the best compromise between volume and performance. It just might work. Oh and use a larger value pot on the input too. (if you have one)

A.

[indigo.girl]

Originally Posted by peter_sol

Most basic tone controls simply reduce the treble content of the audio but with a decent speaker (Try the one in your gram) may not be needed. Do you have any pots and capacitors around to try?

Thanks Peter - I'll have a go with some better speakers and see how it sounds. Although I'm a bit worried about using the high output TC8H with it now in case its too much for my op amp...

[ms660]

Originally Posted by indigo.girl

Ah, so are you and Andy saying the 430K resistor in series with the input channel is causing me problems? I'm getting a little confused as I thought the 430K I've added here increases the impedance of the IC circuit from 20K to about 500K hence matching the requirements of this Piezo cart? Can I wire it differently to increase the impedance without causing the 'attenuator' problem?

You have a cartridge, it has an internal impedance, the output from the cartridge is connected across two resistors in series which comprise of the 430k resistor and the two input resistors and volume controls which are connected in a series parallel arrangement that equal a resistance of 10k, according to the pictures.

That gives a total resistance across the cartridge of 440k, no problem with that, we can view that as a high impedance load across a high impedance cartridge.

The 430k and 10k form a potential divider, the IC's input is taken from that potential divider, the ratio of the two resistances is 43:1, the voltage ratio between the input and output of that divider will also be 43:1, eg: 100 millivolts input to the divider will give 2.33 millivolts out (approx) and that's assuming the IC's input impedance is infinite, which it won't be, the IC's input impedance will reduce the output from the potential divider even further but probably by not a lot.

Hope this helps to explain things.

Lawrence.

[bikerhifinut]

https://www.vintage-radio.net/forum/...d.php?t=113530

Thats the thread on the Module.

By the way, your little power amplifier chip isn't an op amp although its likely that theres an op(erational) amp(lifier) inside the chip as part of the overall circuit.

I think maybe we are bombarding you with info that may be just slightly ahead of your current skill set? I may have made the error of assuming you understand building blocks of theory like potential dividers etc? In which case apologies.

But you are learning fast I think?

A.