Lab Series L5 - noisy

[Jacs]

Hi,

I just became the happy owner of an L5 and I really like the sound
both for guitars and keyboards including my Yamaha CP 70. The reverb
tank is missing, though but I guess I could fit in an Accutronics
can.

My only concern is the noise level.

All users out there seem to praise the quietness of the L5, but mine
is very noisy:

I get "Hum" on both channels and notably from the master volume when
approaching 12 o'clock and beyond.

I also get high frequency hiss/white noise when turning up treble and
choosing "hi-pass" like settings on the frequency/midrange knobs.

I would greatly appreciate any guidelines for diagnosing and solving
this problem.

Best regards
Jacob // Copenhagen, Denmark

PS. It's a model with the "normal" on/off power switch on the back
like the one pictured here: http://www.guitare-village.com/occasion/amplis/amplis_142.php

[mw]

Post by Jacs
Hi,
I just became the happy owner of an L5 and I really like the sound
both for guitars and keyboards including my Yamaha CP 70. The reverb
tank is missing, though but I guess I could fit in an Accutronics
can.

Does the "clean side" sound different than the adjustable side (the left
side that has the reverb control)? If so, you might investigate the
input connection from the missing tank. Maybe it is making accidental
contact with some part of the chassis. Also, try your own suggestion of
using an alternate tank.

Post by Jacs
My only concern is the noise level.
All users out there seem to praise the quietness of the L5, but mine
I get "Hum" on both channels and notably from the master volume when
approaching 12 o'clock and beyond.

My L7 is about 30 years old. I don't hear significant hum (other than
the usual guitar pickup hum).

Post by Jacs
I also get high frequency hiss/white noise when turning up treble and
choosing "hi-pass" like settings on the frequency/midrange knobs.

I do get some hiss, but once the guitar is played the sound of the
guitar is far louder, and the hiss is not that bothersome at least to
me. The knob settings you mention will increase hiss, as they do on any
amplifier. The L5 circuit uses older solid state circuitry (like the
OTA amps) that is not as quiet as what might be possible today, so there
will be some hiss.

It sounds like you have either a problem with the reverb tank input or
possibly bad power supply capacitor or rectifier, or related connections.

Hope this gives you some ideas.

[ylo]

On Aug 20, 10:02 pm, mw <***@nospam.com> wrote:

Lab Series hum and hiss:

I have an L5 and have had some issues with hiss and 'motoboating.' I
tracked this down to the compressor / distortion (mixer-distortion)
section of the preamp. Replacing the A109A RC4558 and A111 CA3080E
opamps reduced the noise. Having an oscilloscope helps track down the
hiss. While you are replacing opamps, you might want to put in
sockets in case you have to replace them again or in case you want to
try different pin-compatible substitutes. Some of the newer opamps
may have lower noise, although this does not necessarily mean that
they will sound any more or less musical.

If the hum is not from the reverb circuit, I would check the power
supply. Ripple is supposed to be less than 5 mV, but if you can hear
hum, you may have much more ripple than this, possibly from a bad
electrolytic capacitor. Just like the high voltage ones in tube amps,
the low voltage ones can go bad after 30 years.

Chris Taylor
Boston MA

[mw]

Post by ylo
I have an L5 and have had some issues with hiss and 'motoboating.' I
tracked this down to the compressor / distortion (mixer-distortion)
section of the preamp. Replacing the A109A RC4558 and A111 CA3080E
opamps reduced the noise. Having an oscilloscope helps track down the
hiss. While you are replacing opamps, you might want to put in
sockets in case you have to replace them again or in case you want to
try different pin-compatible substitutes. Some of the newer opamps
may have lower noise, although this does not necessarily mean that
they will sound any more or less musical.
If the hum is not from the reverb circuit, I would check the power
supply. Ripple is supposed to be less than 5 mV, but if you can hear
hum, you may have much more ripple than this, possibly from a bad
electrolytic capacitor. Just like the high voltage ones in tube amps,
the low voltage ones can go bad after 30 years.
Chris Taylor
Boston MA

All this is good advice. Well said.

[RS]

Post by ylo
I have an L5 and have had some issues with hiss and 'motoboating.' I
tracked this down to the compressor / distortion (mixer-distortion)
section of the preamp. Replacing the A109A RC4558 and A111 CA3080E
opamps reduced the noise. Having an oscilloscope helps track down the
hiss. While you are replacing opamps, you might want to put in
sockets in case you have to replace them again or in case you want to
try different pin-compatible substitutes.

Good advice above from Chris. There are lots of low-noise subs for
4558's, so use the socket.

Not many manufacturers making OTA's (Operational Transconductance
Amps). There was a more modern, lower noise version called the CA3280.
That's a dual though, and it's no longer in production. You may be
able to put a small adapter socket together if you can find the part.
They're often grabbed immediately by synth builders. Real popular in
older VCA designs.

The other alternative is the LM13700, but I doubt that the response
characteristic is close.

In any case, the CA3080 is a relatively noisy chip. You may do well to
replace it if hiss is the major problem.