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Need help with Triple Parallel LM1875 dynamics amplifier.
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Old 18th December 2011, 06:34 AM   #91
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by Grega View Post
I interpret this as being the speaker ground is the same as the input ground
I hope not. The speaker return doesn't really belong at inputs, because that makes fiercely loud mids. Got baxandall? If you were going to use a baxandall anyway, well the dynamics of bouncing the amplifier's inputs about can be fantastic although the tone would need fixed a lot. I wouldn't do it that way.

My views on the matter are in the sketch in post #71 above, which is a practical triple parallel on a single board, so easily done and very compact. See the speaker and power on the farthest left side, but the small signal input on the farthest right side?

For a parallel amplifier, I am finding that the discussion of using 1 chip per each board, and then "stacking" was a great deal more confusing and more complicated. That got me lost too.

However, your circuits and layouts have improved dramatically in a very short time. Kudos!

Last edited by danielwritesbac; 18th December 2011 at 06:40 AM.
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Old 18th December 2011, 11:47 AM   #92
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Grega View Post
I don't know what RC and F-3db means but I will try to read up and understand

..............I interpret this as being the speaker ground is the same as the input ground. correct?
sorry for the questions but some of these is well above me.
a filter with just an RC or a CR pair is a single pole 6dB/octave slope low pass, or high pass filter.

The F-3dB is the frequency that the response is down by 3dB from the passband level. The F-3dB = 1 / [2piRC]

Sometimes rather than quoting the frequency, Members and others, will quote the filter time constant. The RC of a filter is that time constant.
eg. FM radio has a 50us or 75us filter.
Vinyl recording RIAA is a combination of RC filters of 3180us, 318us, 75us
Yes, one of those filters is the same as FM radio.

The RC is simply the resistance in ohms multiplied by the capacitance in Farads with the result in seconds (s)
eg 7500ohms (7k5) and 0.00000001F (10nF) {that should read 7 zeros after the leading zero.} gives 0.000075s (75us)
F-3dB = 1 / [2 * 3.14159 * 7500 * 0.00000001] = 1/2/3.14/75/10^-6 = That 75 & 10^-6 is 75us = 2122Hz (2k122Hz)
Since RC is already in the F-3dB equation, you can swap between Hz and seconds by using the factor 6.28 (2PI) and taking the reciprocal (1/X on the calculator) 2122 Hz * 6.28 = 13326. Reciprocal or Invert (1/X) that to get 75us.
And it works the other way. Take 3180us*6.28 and then 1/X = 50.05Hz

Your schematic is correct. The Signal Return (Signal Ground) is separate from the Main Audio Ground (MAG). The Speaker Return must go to the MAG. The SPKR Return must not go to the Signal Return.
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Last edited by AndrewT; 18th December 2011 at 12:06 PM.
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Old 18th December 2011, 08:28 PM   #93
Grega is offline Grega  Australia
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Quote:
I hope not. The speaker return doesn't really belong at inputs, because that makes fiercely loud mids. Got baxandall? If you were going to use a baxandall anyway, well the dynamics of bouncing the amplifier's inputs about can be fantastic although the tone would need fixed a lot. I wouldn't do it that way.
OK
I've heard of baxandall, but, don't know what it is. Will change that

Quote:
My views on the matter are in the sketch in post #71 above, which is a practical triple parallel on a single board, so easily done and very compact. See the speaker and power on the farthest left side, but the small signal input on the farthest right side?
watch this space. i like your layout dan - lets see if i can board it

Quote:
For a parallel amplifier, I am finding that the discussion of using 1 chip per each board, and then "stacking" was a great deal more confusing and more complicated. That got me lost too.
most of this is confusing, i'm getting it squared off in my head which is important - and a great learning experience - so thanks to all

Quote:
However, your circuits and layouts have improved dramatically in a very short time. Kudos!
thanks dan

Quote:
a filter with just an RC or a CR pair is a single pole 6dB/octave slope low pass, or high pass filter.

The F-3dB is the frequency that the response is down by 3dB from the passband level. The F-3dB = 1 / [2piRC]

Sometimes rather than quoting the frequency, Members and others, will quote the filter time constant. The RC of a filter is that time constant.
eg. FM radio has a 50us or 75us filter.
Vinyl recording RIAA is a combination of RC filters of 3180us, 318us, 75us
Yes, one of those filters is the same as FM radio.

The RC is simply the resistance in ohms multiplied by the capacitance in Farads with the result in seconds (s)
eg 7500ohms (7k5) and 0.00000001F (10nF) {that should read 7 zeros after the leading zero.} gives 0.000075s (75us)
F-3dB = 1 / [2 * 3.14159 * 7500 * 0.00000001] = 1/2/3.14/75/10^-6 = That 75 & 10^-6 is 75us = 2122Hz (2k122Hz)
Since RC is already in the F-3dB equation, you can swap between Hz and seconds by using the factor 6.28 (2PI) and taking the reciprocal (1/X on the calculator) 2122 Hz * 6.28 = 13326. Reciprocal or Invert (1/X) that to get 75us.
And it works the other way. Take 3180us*6.28 and then 1/X = 50.05Hz

Your schematic is correct. The Signal Return (Signal Ground) is separate from the Main Audio Ground (MAG). The Speaker Return must go to the MAG. The SPKR Return must not go to the Signal Return.
cool - thanks andrew - thats great.
before reading that I didn't know what RC or CR is. do now
understood - time constant vs freq- horses for courses

i have a question (or three)
so we have two grounds - signal audio return and main audio ground.
is signal audio return the line level input negative? or the speaker negative?
and is main audio ground, the 0v in the 18-0-18 transformer out?

interestingly i built a board for something to do yesterday, based on the layout a few posts back (post 77) and proceeded to fry my test speaker - as i had +26vdc on the line out

not sure whether i didn't connect it correctly but, i reviewed my layout and my board and while it looks fine, i know it doesn't work. one thing of note is that the line input ground and the 0v ground (from trafo) are not connected (kept separate) - in previous builds they have been tied together and have worked fine - hence my questions on the ground(s)

many thanks all.
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Old 18th December 2011, 09:19 PM   #94
Grega is offline Grega  Australia
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oh, AndrewT, just a small correction in this line (for people following this extremely useful thread)

F-3dB = 1 / [2 * 3.14159 * 7500 * 0.00000001] = 1/2*3.14/75/10^-6 = That 75 & 10^-6 is 75us = 2122Hz (2k122Hz) (ie: multiplied by Pi as per your previous equation)
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Old 18th December 2011, 09:26 PM   #95
Grega is offline Grega  Australia
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Quote:
AndrewT
Look at the RC and F-3dB of the two input filters.
now understood. 7hz and 1.5khz respectively. oops.
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Old 18th December 2011, 10:58 PM   #96
gootee is offline gootee  United States
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Quote:
Originally Posted by Grega View Post
oh, AndrewT, just a small correction in this line (for people following this extremely useful thread)

F-3dB = 1 / [2 * 3.14159 * 7500 * 0.00000001] = 1/2*3.14/75/10^-6 = That 75 & 10^-6 is 75us = 2122Hz (2k122Hz) (ie: multiplied by Pi as per your previous equation)
Sorry. AndrewT had it correct the first time. On a calculator, you enter 1 and then divide by each denominator factor, in any order.

The way you wrote it, you would HAVE to enclose the 2*3.14 in parentheses, since the evaluation order is simply left to right, otherwise. But writing it as you did and enclosing it in parentheses would be less efficient, when using a calculator, than AndrewT's version.

The way you wrote it gives 20953.95 instead of 2122.066.

Last edited by gootee; 18th December 2011 at 11:09 PM.
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Old 18th December 2011, 11:51 PM   #97
danielwritesbac is offline danielwritesbac  United States
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Quote:
Originally Posted by Grega View Post
. . .interestingly i built a board for something to do yesterday, based on the layout a few posts back (post 77) and proceeded to fry my test speaker - as i had +26vdc on the line out
My own test speaker has a 3,300uF series cap. This provides the opportunity to measure for DC offset without the consequences to the speaker.

AndrewT will soon mention the light bulb tester, a safety current drop test, so do look that one up.

Sometimes you'll get a rail output like that because it is a typical chip amp failure mode. Most prone to that are DC coupled amplifiers with authentic and also most fakes on any design, since that whole situation is more fragile and should have lower voltage--derating is the #1 major concern in such circumstances.
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Old 19th December 2011, 06:24 AM   #98
danielwritesbac is offline danielwritesbac  United States
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Post 77 has omitted the ground lift resistor. Before using that amp, it would need 2 ohms from in- to 0v. Its the really bright blue resistor in post 76. Without that part, total omission of reference for signal ground would make for unexpected operation.

Last edited by danielwritesbac; 19th December 2011 at 06:26 AM.
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Old 19th December 2011, 06:46 AM   #99
Grega is offline Grega  Australia
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dan - thats cool. the unexpected operation = 26VDC on the speaker output - when you tie the input ground and power supply ground together, the amp (that single i built) performs flawlessly FYI

AndrewT added inverted diodes either side for protection which I will also add.

gootee - thanks. I used Excel to setup a little spreadsheet to calculate the values based on the formula - get the exact same result as what AndrewT got.
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Old 19th December 2011, 10:04 PM   #100
danielwritesbac is offline danielwritesbac  United States
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That would be a great spreadsheet to have handy. Could you post it?
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