phono load resistors

Graeme · Post by **Graeme** » Fri Nov 05, 2010 7:34 pm

Just a quick question.

If trying primary loading for my cart, do i need to replace the 47k in the phono with 1M?

Why do i need the 1M and why isnt this 1M resistor acting as a large loading resistor?

Not sure whether to just take out the 47k and leave it like that or fit the 1M's ive read about elswhere.

(WD phono III clone by the way and 1:10 tx's)

Thanks.

richardcooper2k · Post by **richardcooper2k** » Fri Nov 05, 2010 8:21 pm

loading resistors are connected from signal to earth

so the SMALLER the value of the resistor, the BIGGER the effect it has

by increasing the value of the resistor on the input of your amp i.e. on the secondary; you can place more of the loading on the primary

at least that how i understand it i.e not gospel !

Graeme · Post by **Graeme** » Fri Nov 05, 2010 8:34 pm

Still dont get it.

At the moment i have 10k on the secondary (in the phono)
Means i have 100r seen by the cart.

I want to try loading the primary so ill take out the 10k and put 100r on the primary.

But, if i put 1m on the secondary (to replace the 10k) and 100r on the primary why isnt that loading the cart at 10,100r?

Thats what i dont get.

richardcooper2k · Post by **richardcooper2k** » Fri Nov 05, 2010 8:52 pm

isn't the 10k is in parallel with the 47K on the input to your phono stage ? so the effective load across the secondary is less than 10K

and you would replace the 47k with 1M; not the 10K

Graeme · Post by **Graeme** » Fri Nov 05, 2010 8:57 pm

No, i replaced the 47k with the 10k.

There is only the 10k resistor loading the cart, in the phono stage, loading the secondary.

I'd forgotten about resistors in parallell.

My maths is wrong, but the theory stays.

So, putting 1m in the phono doesnt actually load the cart right?

Is it too high a value to have any effect?

The cart will only see the smaller resistor?

richardcooper2k · Post by **richardcooper2k** » Fri Nov 05, 2010 9:02 pm

relatively it will have a negligible effect

but you need to leave something across the input to your phono stage

but don't ask me why !

Graeme · Post by **Graeme** » Fri Nov 05, 2010 9:04 pm

Ok, i have some 1m's laying around so i can try it out tomorrow.

Graeme · Post by **Graeme** » Fri Nov 05, 2010 9:15 pm

This is a quote from mike, this is whats been nagging me, i just couldnt find the reply earlier.

'I might suggest, take out the 47k's, have the secondaries act as grid bias for the first stages, and put your load resistor at the cartridge end. So it's whatever it should be for that cartridge. Seems the simplest option to me unless anyone else knows different '

Mike, so the input load resistors (47k as standard) are also doing grid bias?

Is it ok to run no resistors here at all and rely on the secondaries as you suggest?
Therefore not needing the 1m resistor.

What are otehr people doing when loading the primary, are you all using these 1M resistors?

Gerry · Post by **Gerry** » Sun Nov 07, 2010 6:31 pm

Graeme

Yes I'm using 1Meg in place of the 47K and loading on the primary.

The reason as I understand it is the 1meg ties the grid to the ground, in effect it negates the secondary loading.

There was a lot on this on the old forum HERE.
Andrew was the first to really play with it and demonstrate it a Witham.

Mike H · Post by **Mike H** » Mon Nov 08, 2010 12:02 am

Graeme wrote:This is a quote from mike, this is whats been nagging me, i just couldnt find the reply earlier.

'I might suggest, take out the 47k's, have the secondaries act as grid bias for the first stages, and put your load resistor at the cartridge end. So it's whatever it should be for that cartridge. Seems the simplest option to me unless anyone else knows different '

Guilty as charged, I did write that.

Mike, so the input load resistors (47k as standard) are also doing grid bias?

No, the secondary winding is.

Is it ok to run no resistors here at all and rely on the secondaries as you suggest?
Therefore not needing the 1m resistor.

I would have thought so. Only reason I can see why not is so it presents some sort of load for the secondary current to go down. Otherwise there's a possibility the freq. response could be uneven.

What are otehr people doing when loading the primary, are you all using these 1M resistors?

What is the turns ratio (or Voltage step-up ratio) of the transformer?

Graeme · Post by **Graeme** » Mon Nov 08, 2010 6:04 pm

1:10

The thread on the WD forum was very informative.

Ill probably try no resistors in the phono, then try with 1M, then try a mix of primary and secondary loading.

Mike H · Post by **Mike H** » Mon Nov 08, 2010 8:48 pm

1:10

In that case if there's 1M on the secondary that will make the primary 10k

Turns ratio ^2 = impedance ratio

Could put 4.7M on the secondary and then the primary will be 47k

Is the cart MC?

Graeme · Post by **Graeme** » Mon Nov 08, 2010 10:16 pm

yes. I have a couple and will hopefully be adding another mc cart to the stable too.

I am generaly aiming for 100r load but want to try down to 50r too.

I can leave the 47k in the phono and fit 100r on the secondary too to give around 45r total if i remember right.

Mike H · Post by **Mike H** » Tue Nov 09, 2010 3:19 pm

OK that was what I was trying to get at, what impedance does the cart need

In that case, 10k on the secondary will get 100R on the primary

So 47k will get 470R on the primary

So will need to add 127R on the primary to get it down to 100

Graeme · Post by **Graeme** » Tue Nov 09, 2010 6:03 pm

Yeah, i found a couple of handy calculators on-line that tell you totals for resistors in paralell/series and the same with caps.

Saves me having to work it out

For now it will probably stay with the 10k secondary load as i think my time will be put towards this attenuator first.

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