Help with input section of Eden bass amplifier

Over the past several months I have been working on improving my Eden WT-500 bass amplifier. The original design was quite noisy and had rather truncated frequency response, cutting the highs at around 12kHz. The modifications have been in effort to reduce noise and improve the performance of the amplifier. Among the changes which I have made were replacing the TL072 op amps, electrolytic coupling capacitors, and altering the gain staging to improve the signal to noise ratio. I employed a pair of AD825s, several LM4562s, and an OPA2132, and used film capacitors instead of electrolytics where possible. If anyone is interested, I would be happy to give more detail on the modifications. The amp sounds much better now!

At the moment, I'm trying to understand what is going in in the input section (pictured below). It appears that the input signal goes directly into a 2.2K resistor which is tied to ground with a 1M resistor, then into a 1µF coupling capacitor which is also tied to ground with a 1M resistor.

I'm hoping someone might be able to explain the function of the 2.2K resistor before the coupling capacitor. Is it perhaps serving as protection for the input of the tube? The SWR amplifier schematics that I've seen send the input straight to the tube. It seems a bit odd to put a 2.2K resistor in line with the input. At the moment, I've removed the first 1M resistor and shorted the 2.2K resistor at the input. According to another schematic that I've seen later revisions saw the removal of the 1M resistor at the input, so I'm guessing that it isn't necessary. This change has not proven problematic from what I can hear.

As they seem somewhat superfluous to me, I'm wondering if there would be any harm in removing the first 1M and series 2.2K resistors from the circuit? Understanding their role might shed some light on possible down sides to removing them from the circuit.

work out what the 47pf 2.2K are about. Lowpass
Use your friend F= 1/ (2 pi RC)

The input 1 meg to to ground to to remove any charge in the 1 uf input cap to min any popping when plugging in

The other 1meg is the grid to ground R

Now I wonder what they are doing with that tube It looks like it is a dressed up cathode follower I see - HV and +HV More of the circuit might help. Is it a tube just to have a tube oin the circuit?

[quote author="Gus"]work out what the 47pf 2.2K are about. Lowpass
Use your friend F= 1/ (2 pi RC)

The input 1 meg to to ground to to remove any charge in the 1 uf input cap to min any popping when plugging in

The other 1meg is the grid to ground R

Now I wonder what they are doing with that tube It looks like it is a dressed up cathode follower I see - HV and +HV More of the circuit might help. Is it a tube just to have a tube oin the circuit?[/quote]

Thanks for the response, Gus. Much appreciated!

Here's more of the input section of the preamp from the revised schematic which has the 47pF capacitor and 1M resistor removed. Please ignore the errant line which passes through right of the circuit.



The tube does seem to be used as a cathode follower to buffer the signal into the first op amp. From what I gather, this topology tends to minimize the coloration from the tube.

There is a discussion of the Eden's tube input circuit here on Google Groups: http://groups.google.ca/group/sci.electronics.design/browse_thread/thread/f04dca5f323c51c7/1c6f29c95145bb01?

It seems to me that putting a large resistor and a capacitor on the input effectively negates the benefits of using a high quality (low resistance, low capacitance) instrument cable. As mentioned above, the 47pF capacitor was absent on my unit. However, since removing R2, the 2.2K resistor on the input, I've found that the passive sound of my instruments has improved relative to the active sound from engaging the onboard preamp (my active instruments have Lane Poor and Q-Tuner passive pickups and also have active/passive switches). It seems that the 2.2K resistor was causing additional loading on the pickups and causing a loss of highs. Removing the 2.2K resistor has opened up the passive sound to where it seems superior to the active sound.