Peavey Ecoustic 112 - Loud Hiss

[TomCarlos]

Friends....

I recently picked up a Peavey Ecoustic 112 Amp (at a bargain price). I think I now know why! I have never heard an amp that has such a loud "hiss" as this amp.

If I run a line level signal into the Power Amp Input, the amp sounds fine. So the problem with in the front end.

I tried jumping the 3 sets of Send/Returns with no change. (Each channel has it's own Send/Return and there is a master Send/Return).
I tried taking each channel's send to a separate amp- noisy.

The amp looks like your typical Peavey amp- a main board for the Power Supply, Plus and Minus Regulators, and the output section.

The inputs are on a single board connected by 3 cables. There is also a coax that goes to an XLR output.

The hiss is definitely more noticeable as you raise the pots on the EQ. The hiss is definitely worse in the Mic channel.

I have seen a handful of posts on other boards describing this problem- but no solutions. I would have a very hard time believing that Peavey would have released this amp to production with this much noise!

I'm not sure what "hiss" looks like on a scope. Since it is present with no signal, I'll start by doing a visual check and then checking voltages on the ICs and such.

Anyone familiar with this amp and seen/heard this level of hiss before?

Thanks, Tom

[Leo_Gnardo]

On the scope, you won't miss it, hiss looks like it sounds.

We had a thread just two - three weeks back with some suggestions on upgrade op amps you might try to cut noise. I don't remember 'zackly where, but I think Enzo had some ideas. I'm sure someone will remind us - thanks in advance!

Rolling off high frequencies helps. And FWIW there are "adaptive filters" for instance Rocktron's "Hush" that can be used in fx loops.

[The Dude]

It does look like the op amps are in sockets, so it might be easy enough to nail it down to which/an op amp if that is the cause by simply removing them one at a time.

[TomCarlos]

Thanks Leo...

I will see if I can disable the Mic Channel then work on the guitar channel. If I can pick up some hints, I'll go stage by stage.

This amp uses the BA4560P on the input board. I will pull the board tomorrow night and start the investigation.

Thanks!

Tom

[Enzo]

Hook your scope to the speaker, turn it up so the hiss is loud. Look at the image. Now turn it off so the scope trace is clean. That was what hiss looks like.

Remember, Any control that has ANY effect on the hiss is after the source of the hiss. Does turning the channel controls down turn the hiss down? or do they have no effect but the master does?

Chances are it is a noisy semiconductor, probably an op amp.

If you have no 4560 ICs, then 4580s will work, or TL072, or whatever dual op amps you happen to have.

[TomCarlos]

Thanks Enzo....

I have spare 4560's and might have some 4580's too. I will check each opamp and clean the sockets as I go.

I'll try to snap some photos for the thread and anyone else who comes across this one.

Tom

[TomCarlos]

Slowly taking a look around... and noticing something. The preamp board and the 3 ribbon cables. I do not see any GND connections coming through the cables- not unless I missed it. I see several large copper areas for the GND and some components tied to GND, but those do not seem to be tied together. Is it possible that each GND connection on the preamp board is coming through the pots or input jacks as they connect to the chassis?

[Enzo]

Most certainly. If I recall, the PV 5150 has six ground systems that are united by their connections to chassis.

To fire up a Classic 30 with the board out of the chassis, it takes almost every clip wire I have to get everything grounded.

[TomCarlos]

Ok.... I gotcha. I will look again but it appears that it is the Input Jacks that connect the grounds to the chassis- not the pots. I'll scrub the chassis and the threads on the jacks to make sure I have a clean connection there. Thanks Enzo.

[oc disorder]

Just looking at the schematic , it appears easy to isolate the 2 channels (Guitar/& Mic) by inserting a shorting jack (tip to sleeve) in the channels efx return.
It appears the return in the guitar channel has to be switched in. Then you may be able to detect which is the noisy channel.
I had a cheap acoustic amp here ages ago which was noisy even with the gain down. It seemed to be a poor design so on consultation with the owner I fitted a master between the pre and power amp.
That did the trick as he could adjust the pre amp up a bit and reduce the master.
Probably not the case with your Peavey but thought the above worth mentioning.

[TomCarlos]

Thanks OC...

The other way to isolate the channels is to pull the ICs on the preamp board. I have been able to reduce some of the hiss by cleaning the chassis where the Input Jacks connect. I am going to work on that further. I need to do the same with the jacks on the back side of the chassis.

I will work on the power amp board first, experimenting with different ICs (in place of the 4560s). I'm thinking there won't be that much improvement there but I want to get it as quiet as I can. I will then work on the Guitar channel, out of the chassis using a hard wire connection back to the main board (see if I can emulate some star grounding).

And because this is a project amp, I want to compare the circuitry to my other Peavey Amps (like the KB60 and PA mixer amps) to see if there are differences in design. I know that the XR600C Mixer amp uses 1/2 an IC for each band of their EQ. The Ecoustic does not (and that is a big source of the hiss). But if there is a way to make a slight improvement to reduce the hiss, I might try it. We'll see.

Interesting stuff ...

[TomCarlos]

Mates....

I want to put a footnote to this thread... I reassembled the Ecoustic 112 and here are my parting notes.

First, I repaired the coax speaker as per my other thread Peavey Ecoustic 112 - 12" Coax Speaker - Blown Tweeter. The speaker isn't the greatest quality for an acoustic guitar. I ran a speaker cable to a nice PA cab and that sure sounded better. Oh well... this speaker is what it is. If I decide to start gigging with this amp, I might do a speaker conversion like I did on my KB60. Check out that thread Peavey KB60 KB60A Speaker Replacement..

I cleaned the chassis where ground connections are made. On the preamp, that board connects to ground through the 1/4 jacks. I got me some emery paper and cleaned the chassis. On the main board, there are standoffs that connect the PC board to the chassis.

I also experimented with the preamp by swapping out the 4560 chip with 4558 and 4580. The 4580 added more hiss. So there was nothing to gain there.

In the thread Bypass Capacitor On Op Amp V+ and V- Pins, there is mention of Rod Elliot;s paper Audio Designs With Opamps. Elliot has a great collection of articles on his web site- newbies (like me), check it out. Anyway, I started looking at different tone control circuits and wondered "why is my Peavey XR600 quiet while this amp has some hiss?" Well, I think alot has to do with the tone controls. In the XR600, each band (after the channels mix) has it's own IC. If the Ecoustic did something similar, it would be less "hissy. To combat the problem, when not using the Mic channel, I bring the sliders on the 5 band all the way down to a minimal setting.

There is one thing I tried- I sat my new Mackie ProFX12 mixer on top of the Ecoustic. I took the Mackie output into the Peavey Pwr Amp in- sounded nice. The Peavey power amp is ok... it's the preamp that is noisy (compared to something like the Fishman Loudbox).

So... there you have it... The older Ecoustic series is a reasonably priced amp. It was a good idea... unfortunately, sound quality didn't seem to be a big priority. For me and my bud (who will be using this amp), it's a nice practice amp and maybe with a nicer speaker setup, it might make it to a coffee shop.

Thanks for the input on this and the other threads.

Tom