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Vintage threads from the first ten years

Interesting EQ idea

6/21/1999 6:42 PM
Ken Gilbert	Interesting EQ idea Perusing through the RDH4 the other day I noticed a very nice EQ circuit that uses 4 triode stages. That's not a small number, but it's not ridiculous either. A few aspects of the design appealed to me: a) The center frequency, Fo, can be adjusted with a dual-ganged 1MB pot. Tried to find one today, and the best I could do is come up with a 2MB with a split shaft (I'm not willing to wait for mail-order ATM). Not the same, but the additional element resistance will actually increase the range of Fo. I'm looking at something like 100-10KHz, which is a HUGE range. b) Another pot, 250KB, adjusts the amount of boost or cut at Fo. Looking at the frequency response at various settings of the boost/but pot, it COMPLETELY alters the entire range of frequencies. Center position is quite linear... at one extreme, the lows/highs are boosted by about 20dB, and Fo is cut to near null. Other extreme gets you a 20dB mid boost at Fo, and almost no bass/treble. With various settings of this knob, you can change the response from a / to a /, if you know what I mean. If you've ever used an amp with a semi-parametric midrange knob, you know how versatile they are. This is damned close to that. I don't have the RDH4 handy right now, so I can't tell you what page it's on--it's somewhere in the tone controls section. The design is cross-coupled, so it takes a single ended input and gives a PP output, with very good phase inversion. I tried pumping the ouput into a small 5K PP tranny, but there wasn't enough drive, and the output was anemic--too much step-down. Fed into a couple of output tubes I think it would scream. As it stands, I got rid of the OPT, and just went RC to an XLR jack, for plugging into a power amp. Initial tests were carried out varying two 1MB pots, and the circuit worked well. Tube choice was 2 6SN7's. The circuit in the RDH4 used a 6SN7 and a 6SL7, but it seemed to adapt well to the slightly lower gain. I get plenty of swing at the output, and the lower plate impedance of the 6SN7 makes for a better driver anyway. I'll post again with page numbers, and if folks who DON'T have a copy of the bible are interested I'll see if I can draw one or scan it in. This also freed up a 9 pin socket, so I think an FX loop is going in there, something like KOC's. The two 6SN7's went in where 2 EL34's came out, so the DC heater supply is happier as well. The amp used to be capable of about 30W out, but now it's relegated to strictly preamp duty, which was all I used it for anyway. Anybody need an OEM Marshall JCM900 100W OPT? ~KG~

6/21/1999 6:42 PM

Ken Gilbert

Interesting EQ idea

Perusing through the RDH4 the other day I noticed a very nice EQ circuit that uses 4 triode stages. That's not a small number, but it's not ridiculous either. A few aspects of the design appealed to me:

a) The center frequency, Fo, can be adjusted with a dual-ganged 1MB pot. Tried to find one today, and the best I could do is come up with a 2MB with a split shaft (I'm not willing to wait for mail-order ATM). Not the same, but the additional element resistance will actually increase the range of Fo. I'm looking at something like 100-10KHz, which is a HUGE range.

b) Another pot, 250KB, adjusts the amount of boost or cut at Fo. Looking at the frequency response at various settings of the boost/but pot, it COMPLETELY alters the entire range of frequencies. Center position is quite linear... at one extreme, the lows/highs are boosted by about 20dB, and Fo is cut to near null. Other extreme gets you a 20dB mid boost at Fo, and almost no bass/treble. With various settings of this knob, you can change the response from a / to a /, if you know what I mean.

If you've ever used an amp with a semi-parametric midrange knob, you know how versatile they are. This is damned close to that.

I don't have the RDH4 handy right now, so I can't tell you what page it's on--it's somewhere in the tone controls section. The design is cross-coupled, so it takes a single ended input and gives a PP output, with very good phase inversion. I tried pumping the ouput into a small 5K PP tranny, but there wasn't enough drive, and the output was anemic--too much step-down. Fed into a couple of output tubes I think it would scream. As it stands, I got rid of the OPT, and just went RC to an XLR jack, for plugging into a power amp.

Initial tests were carried out varying two 1MB pots, and the circuit worked well. Tube choice was 2 6SN7's. The circuit in the RDH4 used a 6SN7 and a 6SL7, but it seemed to adapt well to the slightly lower gain. I get plenty of swing at the output, and the lower plate impedance of the 6SN7 makes for a better driver anyway.

I'll post again with page numbers, and if folks who DON'T have a copy of the bible are interested I'll see if I can draw one or scan it in.

This also freed up a 9 pin socket, so I think an FX loop is going in there, something like KOC's. The two 6SN7's went in where 2 EL34's came out, so the DC heater supply is happier as well. The amp used to be capable of about 30W out, but now it's relegated to strictly preamp duty, which was all I used it for anyway.

Anybody need an OEM Marshall JCM900 100W OPT?

~KG~

6/29/1999 6:38 PM
Charlie Escher	Ken, Very interested, please post or Email if you get a chance to draw it up. Thanks, Charlie

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