Phono 3 Trimmer

[craigtone]

I am looking to set the trimmer cap on my Phono 3. I understand you inject a square wave and adjust for best waveform. I am curious what frequency and amplitude? I don't want to overload the stage. As far as frequency, does it matter which corner I am tuning and should I match that frequency? That is the 75us corner (2.122KHz), correct?

[Richard]

Hi craigtone and welcome.

The test/adjustment set up is,

Sig gen > inverse RIAA filter > Phono3 > scope loaded with 50K (to mimic pre-amp)

Set the scope input to display 1 or 2V p-p sine wave at 1KHz. Increase the sig gen level to display it on the screen. Switch to SW and check/adjust the trimmer cap for the best corner appearance at 1KHz, and/or try 5KHz if you wish, but it is the high harmonics which are making up the SW corner so 1KHz should be fine. It is very obvious and you will see it over/under shoot as you turn the trimmer.

Given a -40dB inverse RIAA filter such as the Hagerman; the sig gen puts out a flat signal approx 1V > the inverse RIAA filters it to the shape of a cartridge output and attenuates it -40dB to mimic a cart > Phono3 amplifies it +45dB and filters it back to a flat signal > the scope displays what should then be a flat signal again at approx 1V. That way Phono3 sees approx MM cart voltage on its input and puts out a level well within its 5V capability.

[craigtone]

Excellent description ... thanks Richard!

[Richard]

You're welcome, the same hook up and settings can be used to check Phono3 RIAA accuracy by sweeping sine frequency from the sig gen whilst watching for any deviation from flat on the scope. It should just show roll-offs at each end of a flat response. Be careful to use low capacitance coax to hook up, such as good hifi i/cs, so as not to roll off the highs inadvertently.

The inverse RIAA filter can also be checked for accuracy against the Hagerman literature ref 1Kz sine but it should be fine if carefully built.

[Riggers]

Hi all
A bit late to the party on this issue but last weekend I had the chance to test my phono 3 with a Hagerman inverse riaa filter. Interestingly having built the unit 7 years ago and at the time set the trimmer caps to halfway open as recommended, I discovered that the square wave response was almost spot on as originally set. I only made the slightest tweek. Must be why I have had so much enjoyment from my WD gear over the years.
Cheers
Phil.

[TimN]

Originally Posted by Richard

Hi craigtone and welcome.

The test/adjustment set up is,

Sig gen > inverse RIAA filter > Phono3 > scope loaded with 50K (to mimic pre-amp)

Set the scope input to display 1 or 2V p-p sine wave at 1KHz. Increase the sig gen level to display it on the screen. Switch to SW and check/adjust the trimmer cap for the best corner appearance at 1KHz, and/or try 5KHz if you wish, but it is the high harmonics which are making up the SW corner so 1KHz should be fine. It is very obvious and you will see it over/under shoot as you turn the trimmer.

Given a -40dB inverse RIAA filter such as the Hagerman; the sig gen puts out a flat signal approx 1V > the inverse RIAA filters it to the shape of a cartridge output and attenuates it -40dB to mimic a cart > Phono3 amplifies it +45dB and filters it back to a flat signal > the scope displays what should then be a flat signal again at approx 1V. That way Phono3 sees approx MM cart voltage on its input and puts out a level well within its 5V capability.

Hi Richard

About to have a go at this myself.

How do you load the scope with 50K? Presumably it would be 100K if you had a 100K pot in the pre?

[John Caswell]

Hi Tim,
Although Richard says load the output with 50K (or 100K if that is your preamp input impedance) from my tests, in practice it will not actually matter. The input impedance of most scopes is 1M//10pF, the output impedance of the phono stage is about 4K so loading is really immaterial.

John

[TimN]

Thanks John