How to reduce the gain in a Pre 2

[psxmad2]

Having changed some time ago from a passive pre to the WAD 2 pre amp I have put up with the very limited range of useable volume on the pre 2. To this end I wish to reduce the gain of the pre by approx 20 db. Is there a straight forward way to do this ? and if so can someone give me an easy step by step guide and component list to do the job. I am happy to solder and understand the risks but no nothing about electronics. Thanks in advance .

regards
Tony

[John Caswell]

Hi Tony,
Are you sure you mean 20dB? That is a reduction in gain of 10:1.
As the gain of PreII is 5 you must have a very sensitive amp following on or excessive input levels. The shunt pot mod can, by juggling Rs, give you any gain reduction you fancy but I would check that your figure is correct first of all.

John - Dr John

[psxmad2]

Hi, thanks for the reply, I`m not sure about the amount except that if I use a 14db attenuator on the input to the power amps it improves the control a lot but even then its only over the first half of the volume movement. The speakers ore Lowther horns and the amps are Bluebell mono 300b`s

[pre65]

Has your volume pot been "shunted" ??




Philip

[psxmad2]

Hi, No no mods at all, will this reduce the gain ?

[Richard]

Originally Posted by psxmad2

Hi, thanks for the reply, I`m not sure about the amount except that if I use a 14db attenuator on the input to the power amps it improves the control a lot but even then its only over the first half of the volume movement. The speakers ore Lowther horns and the amps are Bluebell mono 300b`s

Hi Tony,

Very sensitive speakers!

Yes, the shunt pot is possibly the best way to do what you ask as it improves the standard pot and attenuates at a good position in the connection. I use it to get a reduction of -10.5dB with Pre3 and it works very well indeed.

Pre2 had +14dB gain and Pre3 has +15.5dB. So, given that you've now tried it with (effectively) no gain a further reduction of 6dB seems a sensible next trial.

See the bit extra now in the FAQ with a couple of pics and some suggested values. I'd start with the -20dB setup and vary the resistor if needed. For example going to a 68K instead of the 91K will give -17.8dB or a 56K will give -16.4dB etc. ;

http://wduk.worldomain.net/forum/sho...45&postcount=1

If your wires are thinner/easier you may just be able to wire them and the resistors directly without the board.

Rich

[psxmad2]

Thanks Rich, I have printed the link and will get my head around whats needed and have a go. A reductiion of 15 - 20 db will be ideal so the shunt mod looks the way to achieve this.
Cheers
Tony

[andrew ivimey]

This all sounds very peculiar to me.

I particular, why use an active pre at all if the gain is too high?

And in general, all these numbers being swapped around. What is the relationship between gain as a ratio (e.g 5 or 10:1) and dB as a relative increase in SPL.

I don't understand how a '14dB' in line attenuator can be accurately specified; dB relative to what? dB across the audio spectrum?

If the music is giving me a mean level of 0dB on the Vu meter and I turn the volume down 20dB, then I have the 10:1 reduction in volume i.e. it is very quiet indeed. What's the point of having a pre amp to amp things up only to then turn things down? Try the shunt mod as a passive and forget all the electronics you introduce with and active preamp.


And ..... I have two WAD pre2s. When connected up to a scope and 1volt at 1kHz goes in, 2 volts comes out at full volume. How is that a gain of 5?

And .... pre2 into 300b driven by 6SJ7 a la Bluebell into Lowthers is nice and loud should not be out of control. Maybe room acoustics plays a part? What kind of horns? I don't know.

confused of Bedford.

[NealG]

Andrew, from the original WAD article. The 6922 has a mu of approx 32 and the optx a step down ratio of 6:1. So 32/6 = a gain of 5.3.

[andrew ivimey]

Thanks Neal but how does this square with, 'when I put 1volt signal into the preamp I get 2volts out, at full volume, on the scope', with both WAD2pres?