50W Amp project, no more cheapo. - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 29th October 2001, 04:19 AM   #11
haldor is offline haldor  United States
diyAudio Member
 
Join Date: Aug 2001
Location: Columbus, Ohio
Hi downhear,

About shielding a transformer against emitting EMI. This is not as easy as just wrapping foil around it. One of the reasons we like toroidal transformers so much is they don't have near as much of a problem in this area as EI transformers do.

Lay your board out so all your sensitive, small signal components (and circuit traces) are as far from the transformer as possible. If you can, put the transformer in a back corner and put all of the power supply caps, etc, between the transformer and the amplifier board. Make sure you don't rount any of the signal cabling near the transformer either.

Be carefull to not make harnesses with just signal wires, always include return wires (grounds) in the same harness. If the return wire is run along side a signal carrying wire then the area of the loop created by the signal wire and it's return is very small and this cable (or trace) will not be as vunerable to emmisions. If you depend on another harness (or the chassis) for the return connection, then you will have created a much larger loop area which will act as an antenna and cause you grief. And twist (3 turns per inch) all of your harnesses inside the amp, both input and output wiring. This will help reduce the amount of noise the input wiring picks up and how much the output wiring radiates.

When you mount the transformer, leave the wiring long enough so you can rotate it 90 degrees if necessary. Many times you can fix an interference problem by just rotating the transformer.

Phil Ouellette

  Reply With Quote
Old 20th November 2001, 02:49 PM   #12
diyAudio Member
 
Join Date: Oct 2001
Just received my LM3876's today, need to complete my amp in a week. Need to populate my stripboard before I send the dimensions for the metal case. Decided to get a contractor to make it after all.

The EI transformer I got is huge, mounting it on a chopping board for now, with 20,000uF per channel. The power supply would be the easy bit, and I plan to make it as flawless as possible as I would like to test different amp circuits.

Exams are over, and i have free time to go speaker building. Any really cheap but good designs out there? I don't mind low efficiency, but I need good midrange/treble and decent bass response, -3 dB at 50Hz? My budget's $200 or so, including cabinets, but I'll get contractors to make the cabinets too, budgeting $100 on speaker drivers.

My question is, to avoid clipping, how can I connect more than one LM3876 to reduce the load on each device? I'm using ESP's design, with supply at +- 40V, and I'm considering including a switch to run the same signal through 2 seperate amplifier circuits. However, this would take up some space, and use up more heatsink than I would like to. Also, when the signal from the preamp is lowered in volume, and consequently voltage, could the sound be distorted?
  Reply With Quote
Old 20th November 2001, 04:46 PM   #13
diyAudio Member
 
Join Date: Nov 2001
Location: Norway
Regarding Copperfoil:
The "transformer winding factory" got it. If you ask them nicely, they will give u some.
They use it for shielding, and they use the same width for the copperfoil as the height innside the EI. They wind it along (parallell) with the windings. But NOT inside the EI, must be winded outside of the iron. Otherwise this will cause a 1 turn that is shorted, and will get VERY hot!

Regards former trafowinder.
  Reply With Quote
Old 20th November 2001, 05:13 PM   #14
diyAudio Member
 
Join Date: Oct 2001
Hey thanks, will call them up and ask for some. Also, I've been reviewing the circuit over at ESP. Can someone explain the reason why volume pots should be placed at the input instead of controlling gain itself. Secondly, the National docs reccomend 100K on the signal input, in addition to a volume pot in serial, while Elliot went with a 1K resistor. If I were to add a volume pot on the input as suggested by National Semi, should the value still be 10K?

Controlling gain would be more beneficial to audio reproduction IMHO, as lower gain = lower distortion, which is what i've read. (or understood so far at least) lower gain also produces less heat at lower volumes than would a circuit set at high gain. Also, would reducing gain in all the circuits i come across improve the sound?

Trying to streamline Elliot's design as I'll be building 4 copies of the same circuit. Will post diagrams(hand drawn) when I'm finished with it
  Reply With Quote
Old 20th November 2001, 06:49 PM   #15
diyAudio Member
 
Join Date: Feb 2001
Location: Columbia, SC
If you adjust the volume by changing the gain, you'll either end up:
1) Doing it by changing the negative feedback ratio, which will give you different characteristics at different volumes. You could get into stability issues if you're not careful.
2) Changing the operating point of the circuit. Again, you'll end up with different sonic characteristics at different volumes. In extreme cases, you could conceivably burn up a gain stage by running too much current through it.
It's best to diddle with the signal levels.
Yes, as a general rule, lower gain means lower distortion. But this is one of those things that you design in in the beginning. Tubes tend to have lower gain than solid state--something gleefully pointed out by solid state folks. They miss the point. With the lower gain comes lower distortion. It's perfectly reasonable to run a tube circuit with 10dB negative feedback, whereas a solid state circuit will require something on the order of 30-40dB feedback in order to perform as well. Since negative feedback has unfortunate sonic consequences, it's not surprising that many solid state circuits sound dry and lifeless. Yes, it's possible to design a low gain solid state circuit. Yesterday afternoon I was playing with a JFET (2N5457) circuit that only had 15 dB of gain. No, I don't know what the distortion was, I'm not equipped to measure such, but with no feedback at all it was -3dB at 150kHz--good enough for audio.

Grey
  Reply With Quote
Old 20th November 2001, 07:15 PM   #16
Geoff is offline Geoff  United Kingdom
diyAudio Member
 
Join Date: Apr 2001
Location: UK
Downhere

Grey has covered most of the points I was going to make, and I'm not going to get into a discussion with him on the merits of tubes v BJT v MOSFET (wrong thread!). However, I would just like to add one point.

As Grey has said, if you make the feedback ratio variable you can run into stability problems. Also, if the amplifier is non-inverting, the gain cannot be reduced below unity so there will be a limit to how much the volume can be reduced. Gains below unity can be achieved with an inverting amplifier, but then you will need an additional inverting stage if you wish to retain absolute phase.

Geoff
  Reply With Quote
Old 20th November 2001, 07:48 PM   #17
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
Quote:
Originally posted by downhere

My question is, to avoid clipping, how can I connect more than one LM3876 to reduce the load on each device? I'm using ESP's design, with supply at +- 40V, and I'm considering including a switch to run the same signal through 2 seperate amplifier circuits. However, this would take up some space, and use up more heatsink than I would like to. Also, when the signal from the preamp is lowered in volume, and consequently voltage, could the sound be distorted?
You can bridge the LM3876s - this effectively the same as two amplifiers, each driving a speaker of half the impedance. The 3876 is not recommended to drive a load of 4 ohms at +/- 40V supply voltage, so this won't work unless you use 16 ohm speakers.
You can also parallel two of them, so each contributes half the output current (effectively, each driving twice the impedance). This is difficult to do, because the different offset voltages will fight one another. Please read the app note referred to in this thread carefully before deciding:
Bridge/parrallel amp from Ic's (200W)
  Reply With Quote
Old 20th November 2001, 09:00 PM   #18
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
I thought of another approach: you could bi-amp, using a separate amp for each driver (woofer, tweeter). It requires an active crossover, but is probably the best approach for sharing the load among power amplifiers. Not an even split, but still a split.
See the article at the ESP site for more details.
  Reply With Quote
Old 20th November 2001, 11:51 PM   #19
diyAudio Member
 
Join Date: Oct 2001
Yeah, biamping's a good idea, I'm just gonna run 4-8 channels first in case I need some more amplification with 5 speaker and 6 speakers

If I follow Elliot's design to the letter, and add a volume pot, 10K or 20K at the signal input, should I change the gain resistor?
  Reply With Quote
Old 21st November 2001, 04:30 AM   #20
paulb is offline paulb  Canada
diyAudio Member
 
Join Date: Jun 2001
Location: Calgary
A 10K pot seems fine, the input impedance is probably close to the 22K on the input.
No reason to change the gain that I can see.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cheapo Guitar Amp sorenj07 Tubes / Valves 9 9th May 2007 07:28 PM
Gc with cheapo components rs1026 Chip Amps 25 7th November 2004 08:46 AM
Cheapo amp project help pls downhere Solid State 47 28th December 2001 07:05 AM


New To Site? Need Help?

All times are GMT. The time now is 12:39 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2