Mini amp circuit

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Fri Oct 23, 2009 1:08 pm

I have another question to go along with this "experiment".

Would using (2) 10k ohm resistors in line with headphone outputs tied together at the opposite
end provide sufficient isolation/mixing to feed a mono amp?

Code: Select all

Right---10k--\
              |--- mono input
Left ---10k--/
I only have enough to build mono right now and wondered if it would be ok this way.

Thanks,

CeaSaR
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Bob Scott
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Re: Mini amp circuit

Post by Bob Scott » Fri Oct 23, 2009 3:15 pm

10K resistors should work just fine. The resistors have a high enough value so you don't have low impedance headphone outputs shorted to each other, and the resistance values are low enough to drive the input impedance of any amp without losing any significant amount of signal level.

I built a 600 ohm combiner back (300 ohms for each phase of balanced line) when my TV station was playing stereo music videos but TV stereo had not yet been developed. Two of these, one for each 300 ohm phase:

L+<---100---\
. . . . . . . . . |---100--->mono+
R+<---100---/
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Re: Mini amp circuit

Post by CeaSaR » Wed Dec 02, 2009 9:12 pm

Geez, just when I thought I was done with this, I come across this circuit. A bit more simple, but I
think I'll stick with mine, as I have the parts. I might try the bass boost, however, just for kicks.

CeaSaR
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Bob Scott
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Re: Mini amp circuit

Post by Bob Scott » Thu Dec 03, 2009 10:03 pm

CeaSaR wrote:Geez, just when I thought I was done with this, I come across this circuit.
C2 is shown connected backwards.
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Re: Mini amp circuit

Post by CeaSaR » Thu Jan 21, 2010 10:46 pm

I have another question that ties in with this project.

Using CircuitMaker to design/test the amp(s), I had to plot the responses by hand previously. I have now found
the AC Analysis (Bode Plot) and I am wondering if this is what I should use to see how well this design should
perform? All the settings point to this being what I am looking for: Start frequency, stop frequency, # of test
points and sweep type. I set the plot up where the frequency (X-axis) is in magnitude and Log scale, the Y-axis
is in Decibels and linear scale.

If this is correct, then the amp is flat from 20-20kHz +-~0.25 dB! At 10Hz it is only down 1.9 dB! WOW!

While I'm at it, here's another question. When using a CT transformer to get +V and -V, what transformer
voltage rating should I use to get the voltage to +-35 volts? 28V for each half with the standard rectification
and cap filtering for the PS? That'd be a 56 V transformer, with a rather hefty price tag. I could get 2 standard
transformers for less and accomplish the same thing.

Anyway, how do you go about figuring out what voltage transformer you need to obtain a certain DC voltage
in the standard rectification/capacitor filter setup? My inquiring mind wants to know...

Thanks again,

CeaSaR
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Re: Mini amp circuit

Post by sghioto » Fri Jan 22, 2010 6:20 am

It's appx 1.4 X the RMS value of the AC voltage minus a .7 volt drop across the diode if using a half wave rectifier or a 1.4 volt drop using a full wave rectifier circuit. Add the diode voltage drop to the DC voltage required and divide by 1.4

Steve G.

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Re: Mini amp circuit

Post by Robert Reed » Fri Jan 22, 2010 9:51 am

If you are trying to convert tranny AC rms output to DC out put using capacitor input rectified network either full wave or full wave bridge, the the tranny's current rating will have to be approximately 1.5 times that of the DC current the load will require. If you use a choke input from the DC rectification circuit, your DC out put will be the same as the trannys rms value . Choke input circuits are rarely used these days but they do have superior regulation over capacitor input configurations. In regards to AC rms/DC output (with capacitor input) I have worked with many transformers that will only give a 1:1 voltage ratio at full load, while others have given me the traditional 1:1.4 voltage ratio at full load. The rectifier loss at the voltages you are shooting for are insignificant and will be about 0.7 volt PER DIODE, so that a full wave bridge will lose 2.8 volts accumulative.
I am assuming you do not want electronic regulation for that high of a voltage as this would only be for the output stage and drivers. The lower level circuits, especially if using Op-Amps could have their own built in regulator circuit which would require minimal current and thus dissipate much less power across the regulator chip.What exactly do you want in terms of DC voltage and maximum load current from your supply?

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Re: Mini amp circuit

Post by CeaSaR » Fri Jan 22, 2010 7:19 pm

Thanks guys.

So using the 1.4 rule, I would need a 26 V transformer to get 35 V using a bridge rectifier and, say, a 1000 uF cap
for filtering. Do I have that right?

35+1.4=36.4
36.4/1.4=26

What I am thinking of is trying a revised version of this circuit using 2n3055's for output. With a +-35 V supply, it
has the potential to put out around 65 W into 8 ohms. With beefed up predrivers, it should be able to get around
120 W at 4 ohms. This is a heck of a lot more efficient than anything I have found on the net. And that is with an
input of 300 mV! BTW, quiescent current is 20 mA.

I would love to be able to set this up to work as a guitar amp. My son (#1 again, the "musician") is tired of his
Line6 modeling amp and wants to go with just a plain amp and pedals. Says he's looking for that "analog" sound.
Of course he would rather have a tube amp (Engl is his current wish), but no one in this family can afford the
mortgage for one of those.

Thanks again,

CeaSaR

PS - Now all I have to find is a more up to date design software that has more actives available. CircuitMaker is
kinda slim in the pickins, if you know what I mean. Anyone have any suggestions?
Hey, what do I know?

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Re: Mini amp circuit

Post by Robert Reed » Fri Jan 22, 2010 10:08 pm

Caesar
Now that you are thinking in terms of high power, have you considered the "Losmandy Amplifier".
This is beauty in simplicity, low distortion and DC to 25 KHz flat response.It uses very few parts and one Op-Amp. The amp has a builder choice gain of 1-10. But of course any gain or frequency juggling could be accomplished with the other half of a TLO82. Also uses 2N3055s in the output stage.Can be built with single or split supply. Split supplys do not seem to be in vogue these days, But I really prefer them as it makes design and construction so much simpler. It also gets rid of that big hunkin output capacitor.

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Re: Mini amp circuit

Post by Bob Scott » Sat Jan 23, 2010 4:08 am

CeaSaR wrote:So using the 1.4 rule, I would need a 26 V transformer to get 35 V using a bridge rectifier and, say, a 1000 uF cap
for filtering. Do I have that right?
I would use a cap closer to 10,000uF. With only 1,000uF there will be as much bass signal wasted across the cap as on the 8 ohm load. A 1,000 cap has an impedance of about 8 ohms at 20Hz.
CeaSaR wrote:What I am thinking of is trying a revised version of this circuit using 2n3055's for output. With a +-35 V supply, it has the potential to put out around 65 W into 8 ohms. With beefed up predrivers, it should be able to get around 120 W at 4 ohms. This is a heck of a lot more efficient than anything I have found on the net. And that is with an input of 300 mV! BTW, quiescent current is 20 mA.
Better change those 2N3055's to 2N3773. You have a +-35V supply. During peak output voltage excusions, 70V will appear across the output transistors' from C-E. 2N3055 is only good for 60V max. Use 2N3773's, good for 140V. Make sure all of your diodes, transistors and caps are good for the voltages that they may encounter.

You'll need a big heat sink. Check the power dissipation of your driver transistors. The may need cooling too.

Look here for all the ways to hook up a power transformer. See pages 11-12:

http://www.hammondmfg.com/pdf/5C08.pdf

Note that a choke input design is rarely used anymore. The choke costs Buxx and the circuit needs a bleeder resistor with a minimum load for proper regulation (ie: to keep the voltage down.)

With the bridge rectifier designs, just wire your CT to ground to split the output voltage. That is the design most commercial amps use, bridge rectifier and a CT transformer, two outut caps.
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Re: Mini amp circuit

Post by Bob Scott » Sat Jan 23, 2010 4:19 am

CeaSaR wrote:If this is correct, then the amp is flat from 20-20kHz +-~0.25 dB! At 10Hz it is only down 1.9 dB! WOW!
With the series cap in front of the load? With the 8 ohm speaker connected?
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Re: Mini amp circuit

Post by CeaSaR » Mon Jan 25, 2010 11:17 pm

Robert Reed wrote:Caesar
Now that you are thinking in terms of high power, have you considered the "Losmandy Amplifier".
This is beauty in simplicity, low distortion and DC to 25 KHz flat response.It uses very few parts and one Op-Amp. The amp has a builder choice gain of 1-10. But of course any gain or frequency juggling could be accomplished with the other half of a TLO82. Also uses 2N3055s in the output stage.Can be built with single or split supply. Split supplys do not seem to be in vogue these days, But I really prefer them as it makes design and construction so much simpler. It also gets rid of that big hunkin output capacitor.
Robert,
I did a search for "Losmandy Amplifier" and found maybe 1 or 2 relevant sketches. Is that the one where an opamp drives
a complementary pair? If so, I've seen them and actually gave them a thought.

Bob Scott wrote:
CeaSaR wrote:So using the 1.4 rule, I would need a 26 V transformer to get 35 V using a bridge rectifier and, say, a 1000 uF cap
for filtering. Do I have that right?
I would use a cap closer to 10,000uF. With only 1,000uF there will be as much bass signal wasted across the cap as on the 8 ohm load. A 1,000 cap has an impedance of about 8 ohms at 20Hz.
CeaSaR wrote:What I am thinking of is trying a revised version of this circuit using 2n3055's for output. With a +-35 V supply, it has the potential to put out around 65 W into 8 ohms. With beefed up predrivers, it should be able to get around 120 W at 4 ohms. This is a heck of a lot more efficient than anything I have found on the net. And that is with an input of 300 mV! BTW, quiescent current is 20 mA.
Better change those 2N3055's to 2N3773. You have a +-35V supply. During peak output voltage excusions, 70V will appear across the output transistors' from C-E. 2N3055 is only good for 60V max. Use 2N3773's, good for 140V. Make sure all of your diodes, transistors and caps are good for the voltages that they may encounter.

You'll need a big heat sink. Check the power dissipation of your driver transistors. The may need cooling too.

Look here for all the ways to hook up a power transformer. See pages 11-12:

http://www.hammondmfg.com/pdf/5C08.pdf

Note that a choke input design is rarely used anymore. The choke costs Buxx and the circuit needs a bleeder resistor with a minimum load for proper regulation (ie: to keep the voltage down.)

With the bridge rectifier designs, just wire your CT to ground to split the output voltage. That is the design most commercial amps use, bridge rectifier and a CT transformer, two outut caps.
Bob,
The 1000 uF caps were just an "off the cuff" value for the power supply filtering. I'd definitely put in higher values if
I could find my calculation formulae for them. "Can't remember where I left them."

I used the 2n3055's because my "old, free software" shows them as being the highest power available in it's list of NPN's.
It also says that they are rated for 70 volts. Checking Motorola's spec sheet, 60 V is what I should be using. It's getting
so frustrating not having a more comprehensive list of actives. And there is no listing of 2n3773's. Rats!

As for the voltages, I'm looking for those that will handle the potential, but as stated above, the program is lacking.

I have a fairly large heat sink that I'd use, I think it came off an old car amp some years ago. As for what the other
ones would use, that depends on what they will end up being and what I can get a hold of.

Thanks for the Hammond link. I checked the hookups and the full wave capacitor input load is the one I was going to
use, albeit with the CT ground as you suggested (if I could find one that had the ratings at a price that wouldn't kill
the bank).
Bob Scott wrote:
CeaSaR wrote:If this is correct, then the amp is flat from 20-20kHz +-~0.25 dB! At 10Hz it is only down 1.9 dB! WOW!
With the series cap in front of the load? With the 8 ohm speaker connected?
Yep, 2200 uF. Here's the plot:

Image

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CeaSaR
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Re: Mini amp circuit

Post by CeaSaR » Tue Jan 26, 2010 11:23 am

Robert,

Regarding the Losmandy amp, would Bob's schematics on page 3 of this thread be of that type? I am inclined
to think they are. And yes, they are so much more simple.

CeaSaR
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Re: Mini amp circuit

Post by Robert Reed » Tue Jan 26, 2010 6:02 pm

Ceasar
No its not Bob's circuit which by the way a very unusual one using the op-amp supply terminals to sink/source bias currents. I have only seen that trick used one other time. The Losmandy circuit actually uses a few more components than that one and is a time proven reliable performer which is used a lot in the movie industry. I don't know what level of power you are looking for, but with current op-amps (+/-18 volts max Vd & Vs), the max power out is about 10 watts rms/8 ohms.I built up a pair for my stereo years ago and they put out a clean 50 watts flat from DC to 25 KHz. At that time I purchased my op-amps from OP-AMP LABS in California. They are rated at +/40 volts supply and named 4009. Since this out fit is not a fab house or large retailer, I have to believe the OEM may have an EIA no. available. The highest I have found in recent production are the TLOxx series with a rating of +'22 volts supply (only in some versions). I think maybe a search of some of the older metal can types or some of the RCA CAxxxx series may show up something.
PS with two 2N3055s being used,this is not complimentary output. That requires one NPN & one PNP.

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Re: Mini amp circuit

Post by CeaSaR » Tue Jan 26, 2010 7:45 pm

Robert Reed wrote:... two 2N3055s being used,this is not complimentary ...
I know, it's typically known as quasi-complimentary when a low-power complementary pair is used to
drive a higher power set of NPN's, one hooked up as a Darlington (NPN-NPN), and the other set up as
a standard pair (Darlington style, but PNP-NPN), the name of the setup escapes me at the moment.

Anyway, if you were to move the transistor base(s) to the output of the op-amp, would that be more
like a "Losmandy"? That's the impression I am getting. This also seems to be the way many "digital",
or class D amps, are set up. Either that or I am in mental overload and am mixing to many fleeting
images together. :shock:

CeaSaR

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