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Old 12th June 2010, 11:41 AM   #151
AndrewT is online now AndrewT  Scotland
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Default follow on from post 149

Hi,
I set up the second channel bridge last night. Went through exactly the same procedure as post149 and determined a worst case Vdiff=15.3mV and a best case Vdiff=3.2mV
Not quite as good.

BUT!
there is a big difference between the two.
This time the correction resistor needed to bring Vdiff=0.0mV is on the same side of the bridge as the 0r5. i.e. the 22k on this side leg is too small.
I could leave it like that because the correction resistor of 6r8 is still 0.03% of the 22k.
But because the 22k is too small the output impedance of the Howland would be -ve. The other channel is +ve output impedance.
I don't know what effect this might have on sound, so I am going to make both Howlands have a +ve output impedance.
As I said the correction resistor has a value of 6r8.
The other channel has a missing 1r5 (I did not fit it). If I change the 6r8 to 8r3 then this corrected side will have R8 bigger than the other side. Now both channels are expected to have a Vdiff=~-0.4mV

This can be checked on the circuit board after you have started soldering.

Place all the resistors. Leave out C10, Leave out C30, Leave out IC1, Leave out IC2.
Place a temporary link from the open end of R9 to the empty pin6 of IC1.
Apply the zero volt connection of your test DC PSU to this link.
Apply the +40Vdc to the empty Pin3 of IC2.
Now clip DMM red probe to IC2 Pin 10 and black probe to IC2 Pin9.
Read the Vdiff of the bridge.
Yes, the Howland and the improved Howland actually have a Wheatstone Bridge in the circuit and IC2 reads Vdiff to work out what current to send to the speaker.

I would suggest that both PCB assemblies read the same polarity PIN 9 to 10 and that Vdiff<5mV. The lower the better, to a point.

BUT!
do not go too close to +-0.1mV. In this region tiny differences in temperature on any of the 22k/47k bridge resistors will change the bridge ratio. The Howland could swap between +ve output impedance and -ve output impedance. I don't know if this would be audible or measurable, but I am playing safe and aiming for -0.3mV to ensure that both channels have a similar high output impedance and that both channels stay +ve output impedance over a range of different temperatures on the well spaced 4 bridge resistors.

I am going to go and check that these instructions actually work later today.
Await confirmation after my swim.
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Old 12th June 2010, 11:43 PM   #152
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To serve as inspiration for those of you building amps, here are a few poor photos of my "ultimate BOM" amps. These are not quite identical to the kits recently offered, but they share many of the same parts, and certainly the same basic circuit.

Some elaboration on the pics: as you can see, the trannies are only an inch or two from the power supply side of the boards. I didn't twist any leads, didn't use any shielding on input or output wires, and my amps are completely silent. I mean deep, black silence. No extraordinary measures are needed to achieve these results, other than keeping leads short and not crossing signal wires with power leads.

The enclosures are the cheapest available thin aluminum. The heatsink is nearly as big as one entire wall of the enclosure, mounted on one short wall, coupled through the enclosure wall to the chip with a nylon screw and insulating pad and lots of thermal grease, spread in a skin-thin layer.

The top of the enclosure is the wooden top cut out of an old Yamaha receiver with a very large metal grille for ventilation. Nothing inside these amps ever gets more than warm.

The black material is Sound Coat from Parts Connexion. It also covers about two-thirds of each long wall. It applies directly to the aluminum and damps it very effectively. The transformer is mounted on a Spectra Dynamics Deflex Toroid Support, also from PC.

The Fostex film & foil input cap is just visible, close by the side of the board, wrapped in neoprene foam and attached to the enclosure with a cable tie. The leads can be seen easily because they're wrapped in white teflon plumber's tape.

I used EAR small isolation feet, also from PC, under the enclosure. Despite its thin wall construction, the enclosure and all components seem effectively isolated from internal and external vibration.

The input jack and binding posts are Vampire copper base (not brass) with gold plating. The hook-up wire is OCC solid copper. All connections are soldered directly to the board.

The LED is mounted on the front of the enclosure, connected by the two white wires crossing under the power leads. All four walls of the enclosure contain some element: one long wall has input and output connections, very near that part of the board; the other long wall has only the LED; one short wall has the heatsink; the opposite short wall has fuse, IEC, and power switch, a heavy-duty toggle that "snaps" real nicely when switched.

You can choose to make your enclosures more attractive, but the most complicated arrangement will probably not function any more effectively than this simple, inexpensive one.

I have noticed these amps are very responsive to better power cords. I built my own, and they sound much better than any "standard" molded cord supplied with other equipment.

Good luck with your builds!

Peace,
Tom E
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Last edited by madisonears; 12th June 2010 at 11:47 PM.
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Old 12th June 2010, 11:50 PM   #153
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Madisonears, did you do the matching analysis that AndrewT and others have done, or did you just "stick" them in?
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Old 13th June 2010, 12:15 AM   #154
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I can hardly understand what Andrew is doing or why, much less attempt to do it. I purchased matched pairs of PRP resistors from Parts Connexion (I think it cost $5 extra per value). Uriah's are probably just as well matched, perhaps even better. I checked them on my own crudely constructed Wheatstone Bridge and found them to match very closely.

I am interested to learn if Andrew's procedure yields better sonics. I don't believe the amp could function any more efficiently, as the chips in mine are barely warm to the touch (certainly also a function of heatsink efficiency) after cranking into B&W 802's all day, as they're doing right now. They sure sound sweet.

Guys, just build 'em already and quit overthinking everything. Mauro knew what he was doing, and these better parts make them sound even better!

Peace,
Tom E
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Old 13th June 2010, 09:57 AM   #155
AndrewT is online now AndrewT  Scotland
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confirmed, the instructions work.
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Old 13th June 2010, 03:16 PM   #156
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Hi Madisonears, I like your compact design. Where did you get your enclosure? Also I assume the cap on the switch is to help with turn on/off thump, how do you have that wired.
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Old 13th June 2010, 05:46 PM   #157
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The enclosure is made by BUD, available from most electronics suppliers, model no. AC-408. It's cheap and crude. There is a cover available separately, but I chose to use something I salvaged from a junked receiver. Looking at these crappy boxes, you would never suspect that they contain amplifiers that are nearly the best you have ever heard.

The blue cap on the terminals of the toggle switch was installed to address to tendency of this amp to pass motor-start spikes on the mains. It is not very effective, and I still can hear pops when a large motor, such as my furnace fan, turns on. Now, during summer, no noise at all.

The amp itself is completely silent during turn on and off. That's what the relay is for, and it works very well.

Peace,
Tom E
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Old 13th June 2010, 08:55 PM   #158
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Default 1/2 watt PRP resistors?

Maybe someone else has asked this question; if so, please forgive me.

Five of my resistors are two wide for the holes in the board. One is the big 1 watt and the other 4 are the 1/2 watt PRP resistors. Is there a trick to putting them in without them looking dorky?
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Old 13th June 2010, 09:13 PM   #159
udailey is offline udailey  United States
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They will look dorky. Bend one lead and leave the other straight. Put them in vertical. Bend by grabbing with pliers and bend the lead over the pliers to not stress the joint.
Uriah
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Old 13th June 2010, 09:16 PM   #160
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It's easy enough to shape the legs so that the resistors sit horizontally, albeit just off the board.
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