EMRFD Message Archive 2140

Message Date From Subject
2140 2008-09-28 08:19:02 cwfingertalker LM-386 Bypass Question.
Hello to the Group.

I am building a simple audio amplifier using the LM-386. My question
is about Pin #7 labeled "Bypass".

I did experience some motor boating oscillations and seemed to have
solved that with the usual bypassing methods. I have a 0.01 uf disc
and 220uf electrolytic on Pin #6 (B+) to ground. Also the standard
0.05uf disc capacitor in series with a 10 ohm resistor to ground on
the audio output Pin #5.

Although this cleared up a lot of the problem I still had a noticeable
hiss with no audio input signal. I looked at the LM-386 data sheet
and tried some capacitors from Pin #7 to ground (Bypass). I started
with a 0.05uf then a 1uf and finally a 10uf. The hiss is now almost
all gone.

My question is what is a good value for a bypass capacitor on Pin #7?
Should I use a larger capacitor than 10uf? The LM-386 data sheet
does not seem to give a value or a hint what to use.

Thanks in advance.

Bill N7EU
2141 2008-09-28 13:24:01 Allison Parent Re: LM-386 Bypass Question.
2142 2008-09-28 20:44:29 bobtbobbo Re: LM-386 Bypass Question.
-Bill, you might want to try a more modern part, the TDA7052A by
Philips. It is 1 watt and has a DC volume control, which eliminates
the need for shielded leads to and from a volume control. That family
also has a part, TDA7052, that uses a traditional volume control, if
desired. The data sheet shows a 220 Ufd in parallel with a 100Nfd on
the DC input lead.

Bob, K1AO
2143 2008-09-28 20:44:32 cwfingertalker Re: LM-386 Bypass Question.

Thanks for the tips. Your comments about the high frequency
oscillation confirmed my suspicions. I noted with the scope a
"carrier" super imposed on the AF waveform. Also noted the DC current
in the chip did not vary much with the volume control. Also the chip
just seemed to be running really hot to the touch. I'll do a rebuild
and see if I can chase away the gremlins.

Bill N7EU
2145 2008-09-29 07:43:00 cwfingertalker Re: LM-386 Bypass Question.
Hello Bob.

I did a search in the Mouser online catalog and saw that same chip a
while ago. I think I will order up some the next time a place an
order and try them out. I had some 386's in the junk box and wanted
to whip up a quick solution for a headphone amplifier. Thanks for the
tip. Have you used them in any projects? They sure look simple to
build up. Also I like the "ballanced" output.


Bill N7EU
2146 2008-09-29 08:59:08 Dave Re: LM-386 Bypass Question.
2147 2008-09-29 09:02:24 bobtbobbo Re: LM-386 Bypass Question.
2148 2008-09-29 12:43:12 timshoppa Re: LM-386 Bypass Question.
2149 2008-09-29 21:30:31 cwfingertalker Re: LM-386 Bypass Question.
Greetings the Group.

Thanks for all the useful information. Yes I agree. Future projects
will be with the newer chip (TDA7052A/AT).

Bob. Have you seen the "mute" information in the notes? I wonder if
that could be used in a QSK configuration. Maybe it could be as
simple as a transistor switch connected to the key to drop the voltage
for the volume circuit to zip and mute the audio. Another experiment!

Bill N7EU
2150 2008-09-30 07:04:02 bobtbobbo Re: LM-386 Bypass Question.
2151 2008-09-30 07:38:48 WA0ITP Re: LM-386 Bypass Question.
If needed, an audio taper pot can be simulated from a linear pot and a resistor,
more info here.

I love this radio stuff !
72, 73 Terry, WAƘITP

----- Original Message -----
2152 2008-09-30 09:46:04 timshoppa Re: LM-386 Bypass Question.
2166 2008-10-02 22:22:24 jr_dakota Re: LM-386 Bypass Question.
One thing I've found when using the LM386 is to run it at minimum gain
and it's considerably quieter, if you need more gain use a low noise
op amp in front of it .... Allison's trick of using a cap in the
feedback loop to roll off the highs helps also, especially in designs
where you don't need the full audio range which is most gear we are
making ... Allison's suggestions on properly bypassing the IC is spot
2168 2008-10-03 09:29:17 cwfingertalker Re: LM-386 Bypass Question.

Thanks for tips. I have done a rebuild and the amp is behaving much
better now. Also included all the tips I have read. I started with
the "low gain" version (no caps on pins 1 and 8). The design goal was
to have a low gain headphone amp for portable use. I had a few
LM386's in the junk box and decided to use them up. In future
projects I will use more modern chips.

Thanks again.
Bill N7EU
2174 2008-10-03 18:52:36 jr_dakota Re: LM-386 Bypass Question.
Here's another good article you might want to bookmark/download


I've been using those tricks to make reverse log pots for balanced
microphone preamps for over 20 years .... It can also be used to
linearize the tuning of a varactor diode (Part of the reas