MOTM

> -----Original Message-----
> From: media.nai@... [mailto:media.nai@...]
> Sent: Thursday, July 25, 2002 1:08 PM
> To: motm@yahoogroups.com
> Subject: [motm] simple headphone amp??
>
>
>
> It seems like synth-diy is down again, so I'm posting this here.
>
> I need a simple headphone amp that can run from the RCA outs of a VCR.
>
> The SSM2135 is a dual op-amp specialized for audio and single-sided
> supplies, and is used to drive the headphone outputs in a number of
> commercial products.   Since I already have a SSM2135 in a DIP
> package, I'm
> thinking I could put it on a perfboard in a little project box.  That
> should take me less than a half hour, which is about all the time I have.
> According to AD, it is designed to "tolerate poorly regulated" power
> supplies, so it should be happy with couple of bypass caps and a 9VDC wall
> wart.
>
> Anyway, I have a few questions.  All of the examples of 2135-based
> headphone amps that I could find, both inverting and non-inverting, show
> enormous caps (eg. 100uF-470uF) in series with the output.  Is
> that only to
> prevent DC offset from damaging the headphones??  The bigger the cap the
> better the bass response, so that explains the huge caps, but electrolytic
> caps sound horrible.  I would like to keep them out of the signal path.
> The 2135 has a typical input bias of 300nA.  If I build a
> non-inverting amp
> with an AC coupled input, shouldn't that be sufficient??

Yes, those caps are there to keep DC from frying your headphones or anything
else downstream. You cannot simply AC couple the input and expect there to
be no DC on the output. Look at the data sheet functional diagram and you'll
see regardless of what comes in, the output is still at some DC level. This
device can be powered bipolar, and that might allow you to circumvent the DC
blocking caps.

> Also, what is the best way to add a volume control -- the input??  the
> output?? the feedback path??  I'm thinking that a divider at the output
> might be best, that way I can fix the input impedances to minimize offset
> with fewer resistors.  Also, the output impedance of a headphone amp
> doesn't matter much.  Any thoughts??   Mouser sells a Xicon dual-ganged
> 24mm 1/4W audio-taper 1K pot with solder lugs for $2.35.  That was the
> lowest value dual audio pot I could find.  I'd rather have conductive
> plastic with a lower value and I higher wattage rating, but what
> can I do??
>
> Is there any trick to use a single pot, or do I need to buy a dual pot??
>
> Sorry, I haven't had any time to think about tube filters.

It seems like synth-diy is down again, so I'm posting this here.

I need a simple headphone amp that can run from the RCA outs of a VCR.

The SSM2135 is a dual op-amp specialized for audio and single-sided
supplies, and is used to drive the headphone outputs in a number of
commercial products.   Since I already have a SSM2135 in a DIP package, I'm
thinking I could put it on a perfboard in a little project box.  That
should take me less than a half hour, which is about all the time I have.
According to AD, it is designed to "tolerate poorly regulated" power
supplies, so it should be happy with couple of bypass caps and a 9VDC wall
wart.

Anyway, I have a few questions.  All of the examples of 2135-based
headphone amps that I could find, both inverting and non-inverting, show
enormous caps (eg. 100uF-470uF) in series with the output.  Is that only to
prevent DC offset from damaging the headphones??  The bigger the cap the
better the bass response, so that explains the huge caps, but electrolytic
caps sound horrible.  I would like to keep them out of the signal path.
The 2135 has a typical input bias of 300nA.  If I build a non-inverting amp
with an AC coupled input, shouldn't that be sufficient??

Also, what is the best way to add a volume control -- the input??  the
output?? the feedback path??  I'm thinking that a divider at the output
might be best, that way I can fix the input impedances to minimize offset
with fewer resistors.  Also, the output impedance of a headphone amp
doesn't matter much.  Any thoughts??   Mouser sells a Xicon dual-ganged
24mm 1/4W audio-taper 1K pot with solder lugs for $2.35.  That was the
lowest value dual audio pot I could find.  I'd rather have conductive
plastic with a lower value and I higher wattage rating, but what can I do??

Is there any trick to use a single pot, or do I need to buy a dual pot??

Sorry, I haven't had any time to think about tube filters.

I quickly looked at that page and the reason they don't use DC blocking caps
is because those are all bipolar supply circuits. They just don't happen to
show the power...

What they are talking about is adjusting for offset errors, not power DC
offsets.

> -----Original Message-----
> From: media.nai@... [mailto:media.nai@...]
> Sent: Thursday, July 25, 2002 3:26 PM
> To: motm@yahoogroups.com
> Subject: Re: [motm] simple headphone amp??
>
>
> At 2:31 PM -0500 07/25/02, Paul Schreiber wrote:
> >
> >You need the output caps. Just bypass the electrolytics
> > with a 0.1uf film cap.
>
> Thanks, I've seen that trick used for crossovers, but I guess I forgot.
>
> If I use a non-inverting amp, should I still AC-couple the inputs??  Or am
> I better off using a DC-coupled inverting amp??  I've read an inverting
> headphone amp made from the same op-amp will be noisier, but it will use
> much fewer parts.
>
> >Use a dual gang, 25K or 50K log pot on the *input*.
>
> OK :)  Mouser sells 50K dual ganged log pots.
>
> At 12:36 PM -0700 07/25/02, Tony Karavidas wrote:
> >
> >Yes, those caps are there to keep DC from frying your headphones or
> >>anything else downstream. You cannot simply AC couple the input and
> >expect there to be no DC on the output. Look at the data sheet functional
> >>diagram and you'll see regardless of what comes in, the output is still
> >at >some DC level.
>
> That makes perfect sense, but you know what is odd??  I found this site:
> http://headwize.com/projects/opamp_prj.htm
> and _none_ of the op-amp circuits have output caps.  According to that
> site, offsets can be sufficiently minimized by using the right feedback
> resistors.
>
> Yet, all of the commercial applications of the 2135 that I found used
> output caps.  So did all of the examples in the AD's application notes.
> Two out of two professional EE's on this list agree.  So I guess, what is
> possible in theory isn't safe enough in practice.
>
> >This device can be powered bipolar, and that might allow you to
> circumvent
> >>the DC blocking caps.
>
> It can, but I chose it because it can be run single-sided.  That, and
> because I already have a few.
>
>
>
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>

At 2:31 PM -0500 07/25/02, Paul Schreiber wrote:
>
>You need the output caps. Just bypass the electrolytics
> with a 0.1uf film cap.

Thanks, I've seen that trick used for crossovers, but I guess I forgot.

If I use a non-inverting amp, should I still AC-couple the inputs??  Or am
I better off using a DC-coupled inverting amp??  I've read an inverting
headphone amp made from the same op-amp will be noisier, but it will use
much fewer parts.

>Use a dual gang, 25K or 50K log pot on the *input*.

OK :)  Mouser sells 50K dual ganged log pots.

At 12:36 PM -0700 07/25/02, Tony Karavidas wrote:
>
>Yes, those caps are there to keep DC from frying your headphones or
>>anything else downstream. You cannot simply AC couple the input and
>expect there to be no DC on the output. Look at the data sheet functional
>>diagram and you'll see regardless of what comes in, the output is still
>at >some DC level.

That makes perfect sense, but you know what is odd??  I found this site:
http://headwize.com/projects/opamp_prj.htm
and _none_ of the op-amp circuits have output caps.  According to that
site, offsets can be sufficiently minimized by using the right feedback
resistors.

Yet, all of the commercial applications of the 2135 that I found used
output caps.  So did all of the examples in the AD's application notes.
Two out of two professional EE's on this list agree.  So I guess, what is
possible in theory isn't safe enough in practice.

>This device can be powered bipolar, and that might allow you to circumvent
>>the DC blocking caps.

It can, but I chose it because it can be run single-sided.  That, and
because I already have a few.

At 2:44 PM -0700 07/25/02, Tony Karavidas wrote:
>
>I quickly looked at that page and the reason they don't use DC blocking
>>caps is because those are all bipolar supply circuits. They just don't
>>happen to show the power...

I'm not sure what you mean here.  I'm talking about a DC voltage offset at
the output due to the input bias current and input offset current (my
understanding is that the inputs must draw at least some current and that
they are not perfectly symmetrical).  Afaik, whether or not the DC source
resistance seen by the input terminals is the same is not dependent on the
type of power supply used.  Then again, I'm not an EE and I could be wrong.

>What they are talking about is adjusting for offset errors, not power DC
>offsets.

Right, although I don't think I know what a power DC offset is.  I was
talking about adjusting the feedback and input resistors to minimize
offsets at the inputs.  Yet, all the examples shown by AD and commercial
manufactures using the chip; either single sided, bipolar, or using a third
half of a 2135 to imitate a split supply; use large caps in series with the
output when used to drive headphones or small speakers.  Paul also seems to
agree with the need for such a design.  So perhaps the adjustments for
offset errors discussed on the website are insufficient in practice.

Btw, it looks like sdiy is back up.

Your original question was something like 'do I need those big caps that
don't sound good...this other site seems to avoid them by using proper
resistors...' or something like that.

What I was telling you is there are no DC blocking caps shown on that site
because those circuits most likely have +/- power. The quiescent output is
nearly (should be) 0 volts.

> -----Original Message-----
> From: media.nai@... [mailto:media.nai@...]
> Sent: Friday, July 26, 2002 5:47 AM
> To: Tony Karavidas
> Cc: motm@yahoogroups.com
> Subject: RE: [motm] simple headphone amp??
>
>
> At 2:44 PM -0700 07/25/02, Tony Karavidas wrote:
> >
> >I quickly looked at that page and the reason they don't use DC blocking
> >>caps is because those are all bipolar supply circuits. They just don't
> >>happen to show the power...
>
> I'm not sure what you mean here.  I'm talking about a DC voltage offset at
> the output due to the input bias current and input offset current (my
> understanding is that the inputs must draw at least some current and that
> they are not perfectly symmetrical).  Afaik, whether or not the DC source
> resistance seen by the input terminals is the same is not dependent on the
> type of power supply used.  Then again, I'm not an EE and I could
> be wrong.
>
> >What they are talking about is adjusting for offset errors, not power DC
> >offsets.
>
> Right, although I don't think I know what a power DC offset is.  I was
> talking about adjusting the feedback and input resistors to minimize
> offsets at the inputs.  Yet, all the examples shown by AD and commercial
> manufactures using the chip; either single sided, bipolar, or
> using a third
> half of a 2135 to imitate a split supply; use large caps in
> series with the
> output when used to drive headphones or small speakers.  Paul
> also seems to
> agree with the need for such a design.  So perhaps the adjustments for
> offset errors discussed on the website are insufficient in practice.
>
> Btw, it looks like sdiy is back up.
>
>
>
>