Kawai EQ-8 repair and upgrade

[groovara]

Hi all, I was able to get my hands on the Kawai EQ-8 eight channel parametric equalizer really cheaply. This is my first hardware parametric so I am still learning, but I ran some white and pink noise through it with my DAW and looked at the curves on FreqAnalyst, and they look pretty accurate, if not a bit noisy. One channel is a bit too noisy/buzzy compared to the others, even after spraying some contact cleaner into the jacks. This EQ has 8 separate channels with individual ins and outs, and they can be used in pairs if you plug into the input of one and use the output of the adjacent one. I am planning to use this as a 2-channel EQ with 4 bands each, by connecting the output of one pair via cable and feed it into the input of the next pair.

I am planning to recap this to hopefully remove the noise, and perhaps change the opamps (it uses the Hitachi HA-17558 for the most part and a couple of NJM2043 in the channels with Mic gain options), but I want to check with you guys first, since I am very green with electronics repair. I've done a bit of soldering making my own cables and repaired a few potentiometers, but this will be my first time to totally recap and perhaps change some opamps. I've read a number of posts on opamp upgrades and how it may not be beneficial especially if you do not know what you are doing, since slew rate is not the only consideration. Based on my research this would be considered a state variable filter type EQ.

What would you guys recommend so that I could improve on this? I am open to experimenting and learning on this, since I got it so cheaply. I was able to obtain the schematics for your reference. Thank you all.

 

[abbey road d enfer]

This EQ uses an arrangement known as State-Variable Filter. It is quite sensitive to noise of the opamps used.
I couldn't find a datasheet for the MA1755B. Are they in standard DIP8 or SIP8 package? If they are in DIP8, you could replace them with 5532's.
There are even less noisy opamps today, but they are SMD's. However you can get PCB adapters.
Even the NJM2043's that are supposed to be low noise can easily be bettered by 5532's.
But beware that it could stress the power supply.
A significant part of the problem is that the rather weak rail voltages result in poor headroom, which may not allow you to define a correct internal operating level.
In Line position, the signal is attenuated by 28dB, which results in degraded signal-to-noise ratio.
I would try to correct that by increasing R2 to 200k (so the attenuation would be 10 dB) and compensating at the output by making R25 5.1k (in order to give it 10 dB gain).
That would be about in line with the 10dB headroom loss due to the +/-5V rails instead of the usual +/-15.

 

[groovara]

Thanks for your reply Abbey Road. I've attached the data sheet for HA-17558 for your reference. It's a DIP8 package. Regarding the 5532 opamp that you recommend, is this the NE5532? Do you think I need to put some bypass caps if I used the 5532?

I heard about the inadequate voltage issues from a thread in gearspace and was planning to check the voltages. Also I forgot to mention that the Kawai has a wallwart, not sure if that would be the reason behind the voltage issues.

Thanks again!

 

[abbey road d enfer]

Thanks for your reply Abbey Road. I've attached the data sheet for HA-17558 for your reference. It's a DIP8 package.

OK, I had mistaken the "8" for a "B".
Well they are pretty noisy guys, with 10nV/sqrtHz noise voltage density. You may want to use NJM2068 for about 6dB better noise and compatible current draw. LM4562 is even better by almost another 6dB. I have never used it (it's in may "to do" list) but is has a good rep.

Regarding the 5532 opamp that you recommend, is this the NE5532?

Yes.

Do you think I need to put some bypass caps if I used the 5532?

Aren't there any? I must say I haven't specifically looked at this.
Not having them is malpractice. The designer gets a slap on the wrist.

I heard about the inadequate voltage issues from a thread in gearspace and was planning to check the voltages. Also I forgot to mention that the Kawai has a wallwart, not sure if that would be the reason behind the voltage issues.

I think they could have used a 24V wallwart, if safety was their concern, but maybe they already had that one in the parts bin.
You could make it work with a 24V PSU. You would need to change a few parts. C20, 21 & 22 for a higher voltage (35V recommended), C17 for a 50V, R45 for a 4.7k and D2 for a 21V.
It would be possible to raise th rails to a higher voltage but it would involve replacing a lot of parts, for a minor benefit.

 

[groovara]

Thanks again Abbey Road. I'll try the NJM2068's that you recommend. As for the bypass caps, I often see them being added together with new opamps in order to stabilize them. I'll check the schematics to see if they are already in place. As mentioned I am new to all of this and will need to do some research first before I start replacing parts.