Passive pre amp

[TimN]

Originally Posted by bob orbell

Hi Steve, an Alps blue from Matthew is all you need, unless you have exceptional hearing, he does them at a good price ( look at the WD web site ) and they are made in Japan, not the junk found on ebay. Bob

Going by previous posts, any potential audio benefit may be age related.

Also, it could be an opportunity to go remote control if the joints are starting to stiffen up.

[TimN]

Originally Posted by Greg.

You can’t go far wrong with one of these. I’ve owned one for several years and for the price it represents excellent value for money and sounds good too.

https://www.ebay.co.uk/itm/334505161....c101506.m1851

Just ordered one for myself!

[Thenice]

Hi everyone
Nothing profound but l had a leak stereo 30 once it had resistors all over the volume control
Iike those ones you have been showing
I could be mistaken
Maybe you should nick one from one in a loft somewhere
Whatever l wish you well
Frank

[bob orbell]

Originally Posted by TimN

Just ordered one for myself!

I purchased one of these years ago and fitted to my pre III, to be honest, I could hear no difference from the Alps, so I changed to a motorised Alps powered by a kit from Hi Fi collective, still working good, the stepped pot I passed on to Colin Hepburn. Bob

[TimN]

Originally Posted by bob orbell

I purchased one of these years ago and fitted to my pre III, to be honest, I could hear no difference from the Alps, so I changed to a motorised Alps powered by a kit from Hi Fi collective, still working good, the stepped pot I passed on to Colin Hepburn. Bob

The Alps fitted to my 6550 when I bought it was partially melted by a wiring mistake. I replaced it with an AudioNote specimen that has now started to crackle when adjusted. Just a couple of months old!

I think it's a very harsh environment inside the amp. It gets very hot. Do all valve power amps get so hot?

[bob orbell]

Well, any potentiometer would suffer badly if shorted, as for the AN, it must be dirt inside or faulty and yes all valve amplifiers will and must get hot to work, for the record, the cathode must run at around 1200 deg C or hotter. Bob

[bikerhifinut]

Just looked in and saw this post.
With "Passive preamps", an oxymoron if ever there, was one as theres no preamplification,only attenuation, the principal rule of thumb is to have an output impedance of the pot sufficiently lower than the input Z of the following stage. The "Rule of ten" isn't a bad place to start e.g. if its a 100k gridleak on your amp inputthen it isn't unreasonable to use a 10k pot at the frontMany generalisations have been assumed here and in the end you need to remember that at maximum volume you've got your pot resistance in parallel with the amps input resistance, shunted by the output Z of your source. This Could be as low as 100 ohms or less, my old Arcam CD player had an output Z of around 30 ohms as I remember, not unrealistic from an op amp output with a small series "stopper resistor". So logically there isn't a "too low" pot value as long as the source sees a suitable load.
I've had very respectable results from a passive attenuator using a 10k pot into the 1M load of a Valve amplifier.
Anything up to 100k will work well into a 1meg input. Don't get hung up on this.
As to the benefits of stepped attenuators versus a good quality carbon or other conductive film I will not be drawn into the argument. I've tried the various permutations including a high quality stepped ladder attenuator and these days like Bob I'm more than happy with the Alps RK27 or RK127 motorised version potentiometer.
My feeling as ever has been that perceived audio differences are more down to poor/variable channel balance variation.
I choose a motorised pot as default these days, a remote control source selector I find spurious as one has to get up to the system to change a record or CD and if its streaming or via a digital player etc, you are probably using a DAC which likely has its own remote control.
I'll finish this long post with this observation, from experience, the shorter the distance from the potential divider (ie volume control) the better it is, Ideally the volume pot should be as close as possible to the control grid of the first stage/input pin of first stage device.
Active preamplifiers are useful for 2 things, Amplification if your power amp is low sensitivity compared with source volts, and a low impedance drive along long interconnect wires to a remote power amplifier.


Something to think about, what does adding an extra stage of active noise and distortion add when you can stick your volume pot on the grid of your power amp with the same length of wire as that from the wiper to control grid of your active preamp 1st stage?

Just my take on things, I've been doing DIY audio for a while now and these are my personal experiences.

73

Andy

[TimN]

Hi Andy,

This is very interesting and a great post. I have just jumped ahead from Chapter 1 to 7 of my Morgan Jones. However, I also need to read the bits in-between.

Definitely a subject that I will give more thought to and try to understand more fully.