Hum when old BSR connected to new op-amp/speaker circuit - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

indigo.girl · 1st Jun 2016, 8:21 pm

Quote:

Originally Posted by 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.

I think my electronic knowledge is a bit too rusty and letting me down now. I need to read up on attenuation / impedance matching / buffers.... Can anyone recommend a short beginners guide that might help me improve my basic skills wrt audio/amplifiers/speakers etc....

peter_sol · 1st Jun 2016, 8:24 pm

The high output cartridge will not overload the amp as there is a volume control in between.

Herald1360 · 1st Jun 2016, 9:36 pm

A Maplin Stereo opamp preamp in front of either the present 1wpc amp or the Bardwell 10wpc amplifier module will sort out any impedance matching issues if it's configured correctly.

The Bardwell module has the advantage of tone controls and balance adjust already on board, not to mention ten times the grunt!

Edward Huggins · 1st Jun 2016, 11:27 pm

Just to help indiogirl, the BSR TC8M (c. 300mV) only gives about a third of the output of a BSR TC8H (c. 900mV). Did I read earlier you were thinking of a using a stereo cartridge? BE AWARE: Any NOS Stereo cart you buy now will have LESS output than the BSR TC8M. Edward

indigo.girl · 2nd Jun 2016, 9:19 am

Thanks Edward. Its seems there are three ways to go then to boost the audio.
1) I get a pre-amp to boost what goes to my little amp from my lower output cart.
2) I get a different amp with more gain - perhaps the Bardwell module
3) Or I use some bigger speakers so I can get more volume (I doubt its this simple)

Which avenue do you recommend I pursue?

Edward Huggins · 2nd Jun 2016, 10:09 am

In reply - 1: Yes, you could do this, but will not sound any better. 2: A far better option. 3: Don't think "bigger" speakers always mean louder. As I said in an earlier post to you, a 10 watt rated speaker does not sound twice as loud as a 5 watt speaker. It will probably give the same volume. The output from a speaker depends on what power you drive it with and what power handling capability the speaker has. Those small speakers you have will never give a good sound - all they will give is, er, "some" sound. You need a pair of 5" or 6.5" diameter at least. Edward

Nickthedentist · 2nd Jun 2016, 11:46 am

I have an unused Bardwell module that was kindly given to me by a forum member, but which I'm unlikely to ever use. You can have it for the cost of the postage. PM me if you want.

N.

lazythread · 2nd Jun 2016, 12:18 pm

Quote:

Originally Posted by indigo.girl

3) Or I use some bigger speakers so I can get more volume (I doubt its this simple)

Can I chip in here to say there are three significant aspects to consider with speakers.

One is sensitivity. That is how loud will it go given a certain amount of power. I think it's usually measured in decibels when fed 1 watt of power at a particular frequency and measured at a distance of 1 metre. Someone please correct me as I'm speaking from memory.

Two is power handling capacity. That is how much power in watts it can take from an amplifier.

Three is it's impedance, in ohms, which dictates which amplifier you can use with it.

So it is possible to have a speaker which has a sensitivity of 96dB rated at a maximum of 10 watts of power that will sound much louder, than a speaker with a sensitivity of 86dB rated at a maximum of 100 watts of power if both are fed with 5 watts of power and both have 8ohms impedance.

This is a very simplistic explanation and there are others here who can be more specific, but I hope it helps.

indigo.girl · 2nd Jun 2016, 2:17 pm

Thanks Richard - so its the power output from the amplifer thats the crucial thing rather than the max output possible by the speakers. You only need high powered speakers if you have a high powered output

indigo.girl · 2nd Jun 2016, 2:18 pm

Quote:

Originally Posted by Nickthedentist

I have an unused Bardwell module that was kindly given to me by a forum member, but which I'm unlikely to ever use. You can have it for the cost of the postage. PM me if you want.

Thanks Nick - I'll PM you

bikerhifinut · 2nd Jun 2016, 3:42 pm

Nick knows the amps quite well.
So he should be able to advise you of:

1)Its sensitivity, i.e. how many millivolts of signal are needed to drive it to full power.
2) it's input impedance. This may not be a problem, I seem to remember from the original thread that the tone controls come before the volume pot on this amp and therefore the trick of using a high value series resistor for your cartridge might just work. I think Nick's done that so he's the guy to advise here.
If a resistor won't work then fear not as I or Chris or quite a few others will be able to run you through how to adjust a standard maplins stereo preamp to allow this, including setting a suitable gain from unity upwards. I reckon unity gain would be enough.
3)I believe this amp is suitable for speakers from 4 ohms upwards. I think? you change the output caps from 470 uf to 1000uf to make the most of this. Once done it'll drive any loudspeaker to a decent volume. Again Nicks your man to advise.

I have to say you got yourself a heck of a good deal and from what the people who have used it have posted, you may find it to be a good all purpose stereo Amplifier in a usable domestic system.

indigo.girl · 2nd Jun 2016, 4:25 pm

Today I've been reading all about impedance matching so I can understand the comments more completely....

I'm wondering, does using my high impedance TC8M cart with my much lower impedance amplifer cause damage to the cart because the amplifier will be needing (and hence drawing) more power than the cart can supply it? In other words, have I broken my TC8M cart by using it in a mismatched system. It worked at the start and now it doesn't...

indigo.girl · 2nd Jun 2016, 4:35 pm

Quote:

Originally Posted by pwdrive

Yes, it's the old impedance problem again.

At last, I think I've clicked onto what you are all saying. Although the cart needs to be matched with an equally high impedance amplifier (which is achieved by adding in the 470K resistor in series with the amp chip) the result is a massive drop in the voltage the chip sees as most of the voltage output from the cart is dropped across the big resistor instead ... Hence very little output from the chip because the input is now tiny... hence not much signal for the speakers to convert to sound...

So instead of adding in the big resistor to match the impedance of the cart I need to use a 'pre-amp' as a bridge to increase the impedance instead...

Back to the books again now to learn about impedance bridges...

ms660 · 2nd Jun 2016, 5:11 pm

An impedance bridge usually means something quite different to a lot of folks on here (impedance measuring instrument)

Anyways you seem to have gotten an understanding of the problem of using resistors in an attempt to match two different impedances.

The only way for wideband matching without significant signal loss is either via a transformer or an active device such as a valve, transistor, FET, IC etc. A transistor can be configured as an emitter follower, an FET can be configured as a source follower, either of those arrangements can provide a high impedance input and a low impedance output with only a small drop in signal level between input and output.

Lawrence.

Herald1360 · 2nd Jun 2016, 5:44 pm

Quote:

Originally Posted by indigo.girl

So instead of adding in the big resistor to match the impedance of the cart I need to use a 'pre-amp' as a bridge to increase the impedance instead....

Back to the books again now to learn about impedance bridges...

As Lawrence said, looking up impedance bridges won't help.

Looking up "unity gain buffer amplifier" probably would, though.

Strictly speaking a high impedance input low impedance output buffer does have power gain even if its voltage gain is unity.

The Maplin opamp module is easy to modify- just a few resistors to change value or rewire.

indigo.girl · 2nd Jun 2016, 5:54 pm

Quote:

Originally Posted by Herald1360

A Maplin Stereo opamp preamp in front of either the present 1wpc amp or the Bardwell 10wpc amplifier module will sort out any impedance matching issues if it's configured correctly.

The Bardwell module has the advantage of tone controls and balance adjust already on board, not to mention ten times the grunt!

Is this the Maplins pre-amp you were refering to? http://www.maplin.co.uk/p/mindsets-l...fied-kit-n47fl

This uses an NE5532 chip and has an input impedance of just 1K. So I'm presuming with this I'd need to match the cart with a 500K resistor in series like before. This will attenuate the voltage that the pre-amp sees by 500x but this is off-set somewhat by the 100x gain. So in this set up the amp itself will see a reasonable voltage. But what about matching the preamp and the amplifier? The preamp is 1K and the amp is about 20K - or at these low impedance levels is this difference considered negligible?

bikerhifinut · 2nd Jun 2016, 6:08 pm

It's simpler than that Nicola.
As it happens that is the exact module i modded recently to make a microphone preamp so I can confirm its a piece of cake to modify.
The output impedance of the module isnt an issue, a low output Z into a high input Z is in the vast majority of cases the ideal situation.
So what you need to do is alter the feedback network and input resistor, this is a total of 4 resistors for this stereo amp.
The configuration of this is such that the input resistance is set by a resistor in series with the input and the gain is set by the feedback resistor.
The total gain of the circuit is simply the value of the feedback resistor divided by the value of the input resistor.
With me so far?
So for a desired input Z of 470k you use a 470k input resistor, and then replace the feedback resistor with another 470k resistor.
the amplification factor is thus 470k/470k = 1.
I cant remember which resistors are which on that amp I'll need to look at the data sheet. I seem to remember that its full of misprints.
If any problems understanding my clumsy attempt at explanation do ask.

To address your cartridge question, no you won't have harmed your cartridge.

A.

bikerhifinut · 2nd Jun 2016, 6:11 pm

ok thats the data sheet its public domain so i think its ok to show as its a standard application.

So:

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?

Leave the output potentiometers in circuit as you can use them to trim the output if its a tad high.
A.

indigo.girl · 2nd Jun 2016, 6:23 pm

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?

bikerhifinut · 2nd Jun 2016, 6:23 pm

yep its the capacitor values that are misprinted.
input caps c1 and c4 should be the same value and the decoupling caps c2 and c5 should be the same value.
I seem to remember its 1uF input and 100n for the decoupling.
personally i would decouple with 10uF and use the 100n as input caps here as at a high input z a low value coupling cap would be suitable and a non polarised polywotsit would be ideal. It's not critical though.

1st Jun 2016, 8:24 pm	#62
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Hum when old BSR connected to new op-amp/speaker circuit The high output cartridge will not overload the amp as there is a volume control in between.

1st Jun 2016, 9:36 pm	#63
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Hum when old BSR connected to new op-amp/speaker circuit A Maplin Stereo opamp preamp in front of either the present 1wpc amp or the Bardwell 10wpc amplifier module will sort out any impedance matching issues if it's configured correctly. The Bardwell module has the advantage of tone controls and balance adjust already on board, not to mention ten times the grunt! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

1st Jun 2016, 11:27 pm	#64
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,327	Re: Hum when old BSR connected to new op-amp/speaker circuit Just to help indiogirl, the BSR TC8M (c. 300mV) only gives about a third of the output of a BSR TC8H (c. 900mV). Did I read earlier you were thinking of a using a stereo cartridge? BE AWARE: Any NOS Stereo cart you buy now will have LESS output than the BSR TC8M. Edward

2nd Jun 2016, 9:19 am	#65
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit Thanks Edward. Its seems there are three ways to go then to boost the audio. 1) I get a pre-amp to boost what goes to my little amp from my lower output cart. 2) I get a different amp with more gain - perhaps the Bardwell module 3) Or I use some bigger speakers so I can get more volume (I doubt its this simple) Which avenue do you recommend I pursue?

2nd Jun 2016, 10:09 am	#66
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,327	Re: Hum when old BSR connected to new op-amp/speaker circuit In reply - 1: Yes, you could do this, but will not sound any better. 2: A far better option. 3: Don't think "bigger" speakers always mean louder. As I said in an earlier post to you, a 10 watt rated speaker does not sound twice as loud as a 5 watt speaker. It will probably give the same volume. The output from a speaker depends on what power you drive it with and what power handling capability the speaker has. Those small speakers you have will never give a good sound - all they will give is, er, "some" sound. You need a pair of 5" or 6.5" diameter at least. Edward

2nd Jun 2016, 11:46 am	#67
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,845	Re: Hum when old BSR connected to new op-amp/speaker circuit I have an unused Bardwell module that was kindly given to me by a forum member, but which I'm unlikely to ever use. You can have it for the cost of the postage. PM me if you want. N.

2nd Jun 2016, 2:17 pm	#69
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit Thanks Richard - so its the power output from the amplifer thats the crucial thing rather than the max output possible by the speakers. You only need high powered speakers if you have a high powered output

2nd Jun 2016, 3:42 pm	#71
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Hum when old BSR connected to new op-amp/speaker circuit Nick knows the amps quite well. So he should be able to advise you of: 1)Its sensitivity, i.e. how many millivolts of signal are needed to drive it to full power. 2) it's input impedance. This may not be a problem, I seem to remember from the original thread that the tone controls come before the volume pot on this amp and therefore the trick of using a high value series resistor for your cartridge might just work. I think Nick's done that so he's the guy to advise here. If a resistor won't work then fear not as I or Chris or quite a few others will be able to run you through how to adjust a standard maplins stereo preamp to allow this, including setting a suitable gain from unity upwards. I reckon unity gain would be enough. 3)I believe this amp is suitable for speakers from 4 ohms upwards. I think? you change the output caps from 470 uf to 1000uf to make the most of this. Once done it'll drive any loudspeaker to a decent volume. Again Nicks your man to advise. I have to say you got yourself a heck of a good deal and from what the people who have used it have posted, you may find it to be a good all purpose stereo Amplifier in a usable domestic system.

2nd Jun 2016, 4:25 pm	#72
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit Today I've been reading all about impedance matching so I can understand the comments more completely.... I'm wondering, does using my high impedance TC8M cart with my much lower impedance amplifer cause damage to the cart because the amplifier will be needing (and hence drawing) more power than the cart can supply it? In other words, have I broken my TC8M cart by using it in a mismatched system. It worked at the start and now it doesn't...

2nd Jun 2016, 5:11 pm	#74
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Hum when old BSR connected to new op-amp/speaker circuit An impedance bridge usually means something quite different to a lot of folks on here (impedance measuring instrument) Anyways you seem to have gotten an understanding of the problem of using resistors in an attempt to match two different impedances. The only way for wideband matching without significant signal loss is either via a transformer or an active device such as a valve, transistor, FET, IC etc. A transistor can be configured as an emitter follower, an FET can be configured as a source follower, either of those arrangements can provide a high impedance input and a low impedance output with only a small drop in signal level between input and output. Lawrence.

2nd Jun 2016, 6:08 pm	#77
bikerhifinut Octode Join Date: Nov 2013 Location: Penrith, Cumbria, UK Posts: 1,993	Re: Hum when old BSR connected to new op-amp/speaker circuit It's simpler than that Nicola. As it happens that is the exact module i modded recently to make a microphone preamp so I can confirm its a piece of cake to modify. The output impedance of the module isnt an issue, a low output Z into a high input Z is in the vast majority of cases the ideal situation. So what you need to do is alter the feedback network and input resistor, this is a total of 4 resistors for this stereo amp. The configuration of this is such that the input resistance is set by a resistor in series with the input and the gain is set by the feedback resistor. The total gain of the circuit is simply the value of the feedback resistor divided by the value of the input resistor. With me so far? So for a desired input Z of 470k you use a 470k input resistor, and then replace the feedback resistor with another 470k resistor. the amplification factor is thus 470k/470k = 1. I cant remember which resistors are which on that amp I'll need to look at the data sheet. I seem to remember that its full of misprints. If any problems understanding my clumsy attempt at explanation do ask. To address your cartridge question, no you won't have harmed your cartridge. A.

2nd Jun 2016, 6:23 pm	#79
indigo.girl Retired Dormant Member Join Date: Apr 2016 Location: Loughborough, Leicestershire, UK. Posts: 754	Re: Hum when old BSR connected to new op-amp/speaker circuit 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? Last edited by indigo.girl; 2nd Jun 2016 at 6:31 pm.