133 Dalmeny Drive, Prestons NSW 2170, Australia +612-96074650 & 0412-203382

  JLTi Oppo Player        CD & DVD Player Upgrades        JLTi Tube Amps   

World-Wide Distributors
JLTi BDP Oppo Player
Oppo Level 4 Details
JLTi Phono - Back Again
JLTi EL34 & KT88 Amps
What is Terra Firma?
CD/DVD Upgrades FAQ
Contact Us
Elsinore Speakers DIY
Elsinore Project Kits
JLTi EL34 Wins Shootut
About Joe Rasmussen
JLTi Interconnects
JLTi Review & Comment
Tubes & The Gainclones
DIY Tube Gainclone
Joe's FVP Page
Joe's Equipment Photos
Fletcher-Munson Curves
Joe's Photos
Real Vs False Claims?





Even though the NON-Tube version incorporates similar approach, but it actually has more complications.

Two problems: One, keeping feedback constant (not totally possible but acceptable) when we still have a pot directly in circuit. Actually this did not turn out as troublesome as was thought.

The second problem is major: How do we employ the low pass filter to eliminate HF brightness when component values are shifting about and hence also the turn-over frequency of the filter. This will be covered in this installment.

So this is our REAL PROBLEM, the filtering/bandwidth limiting. How do we get our target of –1.25dB at 20KHz? Oh yes we can get it but only when the pot is set at one discrete position (all you need is adjust 1n3* to get it, simple) but as soon as we move the pot position the filter shifts turn-over frequency and our target is lost.

Clearly it has to come down to a compromise. Please note the suggested compromise works better with lower value pot, so 10K is better than 25K and likewise 25K should be better than 50K (max).

Here is the basic recommended circuit using a 25K Pot:

Below is the basic circuit using a 50K Pot:

Below is the basic circuit using a 10K Pot:

They are much the same except using a different value pots means adjusting the cap value after the 2K2 resistor. Look again and you will see what I mean.

This cap value is the most tricky part of this NON-Tube IGC and what I’ve shown is based on getting the desired –1.25dB value when the pot is set at its –20dB setting.

I ran some simulations (aren’t computers great?) for the three different pot values and measured each at –14dB, –20dB and –26dB. This gives us an indication what happens to the HF response over a window of 12dB adjustment in an area (pot position) where you will most like be when listening to a 2V RMS output CD Player.

Caption for all graphs: The middle line in each graph is the desired minus 1.25dB with the cap value shown and pot in –20dB position. The lines above and below is varying -6dB and  +6dB, a total of 12dB variation of the volume control/pot.

They all look similar but the 10K is more stable whereas surprisingly not much difference between 25K and 50K, provided cap value is made. Please note this is only a guide but also a good starting point when building it first time. Just choose the pot and its recommended value.

Here is the FINAL suggestion, find the volume at which you generally listen and make a note of the Volume pot setting. Find an average setting from a reasonable range of CDs (assuming that is your source). Now vary 1n3 (or whatever matches the pot) cap value by listening carefully – or if you have the instrumentation, adjust to minus1.25dB @ 20KHz. In which case you will still be able to further experiment by ear.

Getting Stable Gain:

Let’s examine GAIN variation with the three different pot values. I made the calculations/measurements and came up with these results:

25K: Circuit Gain variation between +31.2dB and +33.3dB. Quite a good result. Over-all recommendation?

10K: Circuit Gain variation between +32.2dB and +33.3dB. This is close to the ideal BUT with one BIG drawback: The 10K Log pot is not really suitable for a Tube source, but if from a direct coupled (also sometimes called DC coupling) like most CD Players, should be OK. Again vary the 1n3* by ear.

50K: Circuit Gain variation between +29.5dB and +33.3dB. This is just acceptable but loads the source the least. Otherwise I don’t see much problem here.

I would not use 100K.

This then sorts out having too much feedback, much lower than the original IGC. Because of this we now revert to normal Log operation. All three pots are acceptable, but 10K load needs to be kept in mind.


Join Us On




Send mail to joeras@vacuumstate.com with questions or comments about this web site.
Copyright © 2003-10 Joe Rasmussen & JLTi
Last modified: Monday June 08, 2015

Just had a terrible thought. If "intelligent design" is unscientific, then who will design our audio equipment?