I often have been asked if I could develop a phonostage based on my circuit philosophy but with variable EQ curves for mono records. I'm starting such a project now. The first one will be a mono phonostage, transformer coupled using two stages with EC8020. The same circuit as the recently presented EC8020 LCR RIAA phonostage.
EQ will be adjustable by two separate selector switches for the bass EQ (turnover frequency and bass boost) and the treble cut (which selects the -3dB corner frequency of the treble EQ). This first variable EQ phonostage will be based on my best sounding phono circuit, using LCR networks. This means that there need to be separate LCR networks for the bass and treble EQ, since these are indivisually selectable. This is no problem since there are no integrated LCR modules available anyways for other curves than RIAA. So the networks need to be implemented discretely anyways.
It seems that there had been a whole lot of different EQ curves before the RIAA standard was established. Trying to catch all possible curves with selectable LCR networks will be impossible since the LCR filters are quite big and expensive. Therefore I need to find a set of bass and treble EQ networks which provide a useful range which should cover a reasonable range of mono records. 4 to 5 settings maximum seem to be feasable for both bass and treble EQ. Since both can be selected individually this will yield quite a view useful combinations.
After some research I came up with these EQ networks:
Bass EQ, turnover frequency and low freq cut:
1. 350 / -
2. 400 / -
3. 500 / 50
4. 500 / 100
With LCR networks it would be impractical to have the bass boost increasing down to 0 as it would require coils of infinite inductance. So for practical reasons there will be a shelf introduced at around 25Hz for settings 1 and 2.
Treble EQ, high frequency cut, -3dB point:
There are other frequencies around. Some of them are quite close to frequencies in the list above, so as an approximation the nearest setting could be used as a compromise.
These are my initial thoughts on the EQ curves. I would appreciate feedback from people with knowledge in this field. Would you consider this selection as being reasonably representative? Keep in mind this is to cover the most mono records of the 40ies and 50ies. Exotic and rarely used curves for really old 78s are not required for this project.
Please email me or leave a comment on the blog with your feedback.