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Old 9th June 2010, 04:24 PM   #11
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I appreciate your help Russ, Neon, Matt and Ian. Your posts here and on other threads have cleared a lot of brainstorming.

Seems like mouser and digikey are all out of a total of 3 to 4 types of caps and 1 or 2 types of resistors with lead times like mid-July and mid-August.

moparman, chokes are optional in simple pp right?

Last edited by imnewbie; 9th June 2010 at 04:34 PM.
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Old 9th June 2010, 05:09 PM   #12
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Quote:
Originally Posted by imnewbie View Post
I appreciate your help Russ, Neon, Matt and Ian. Your posts here and on other threads have cleared a lot of brainstorming.

Seems like mouser and digikey are all out of a total of 3 to 4 types of caps and 1 or 2 types of resistors with lead times like mid-July and mid-August.

moparman, chokes are optional in simple pp right?
OK...
Although it's handy to "one stop shop", you may end up ordering from several sources. So also try: Allied Electronics, AES, Angela, Parts Express and Parts Connection just to name a few more. Don't get too hung up on brands of resistors or caps. Just make sure to use the correct type as called for in the parts list (ie: metal-film resistor; electrolytic capacitor; etc.)

And don't bother with a choke. Built properly, the SP-P is dead quiet without a choke or fancy caps.
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Old 9th June 2010, 05:10 PM   #13
rknize is offline rknize  United States
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The SPP really doesn't benefit much from a choke. Being push-pull, the ripple on B+ is common-mode in the output stage (which means it cancels itself out because it "ripples" both sides at the same time in opposite directions). The input and phase splitter stages are isolated by an additional RC circuit, so their plates are fairly ripple-free. On my sensitive speakers (98dB), I can hear some 120Hz buzz when I put my ear up to the speaker. But in my shop (89dB speaker, IIRC), it is dead quiet.

The SSE and TSE benefit from the extra filtering the choke provides because they are both single ended and therefore don't have very good CMRR (common mode rejection ratio).
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Old 9th June 2010, 05:35 PM   #14
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Default no choke/cap

OK thanks, I just did not want to start building a chassis until I had all the parts, or wish I had more room to add a component later.
suplimental capacitor is not needed either?
thanks guys
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Old 10th June 2010, 02:52 AM   #15
rknize is offline rknize  United States
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It probably wouldn't hurt, since the amp can get into class AB. It's not strictly needed, though.
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Old 10th June 2010, 10:58 AM   #16
Ian444 is offline Ian444  Australia
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I got curious about a choke too, and started doing scenario's with Duncan Amps' PSUD and a C354 choke. It looked good so I bought a C354 choke from Triode Electronics, 1.5H, 200mA and 55 ohms and ran it in series with a 100 ohm resistor to keep the B+ the same as with just the 150 ohm resistor. Surprisingly (to me, a newb) the sound quality suffered, there were slightly less details. I have no idea why, and I took the choke out.

Now, Russ, or anyone, I have one question which has been puzzling me for some time, that is, the 150 ohm resistor between the two power supply caps, R1, is rated at 5W, and drops around 28V or so, so it is running right near its 5W rating. Normally I allow 3 to 5 times the resistor rating to keep a resistor comfortable heat-wise. But R1 in my amp does not appear to be stressed, so where is the faulty bit in my thinking?

Moparman, I am thinking if you want to spend a bit more money maybe put it into output transformers. From the sound I hear from this amp, I think George "fine tuned" this design a lot more than he lets on about (or wrote about), so I think it's safe to build it just as it is.

Ian.
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Old 10th June 2010, 03:14 PM   #17
rknize is offline rknize  United States
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Well the output tubes draw something like 100mA total at idle for all four. Add say 20 more for everything else.

P=I^2 * R

So that's a little over 2W. The voltage across that resistor is not DC, so if you were trying to measure it with a meter it is not going to give you a useful result. It was more like a peak voltage than the average voltage.
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Old 10th June 2010, 06:18 PM   #18
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This is how I will ground. In UL with cathode feedback.
So this ONE wire(green) coming out of ONE input jack and the grounding is DONE? George explained that PT and OPT are grounded on pcb and pcb is grounded via this one green wire. But then he said the green wirings are actually included in the PCB. So no need to run a separate wire from that input jack to IEC? Then, at which physical place of the PCB is wired to IEC ground?

I just want to be sure since the day I hook it up I should be playing it, not playing a harp with a halo on head. I'm almost chickening out now.

Click the image to open in full size.

Last edited by imnewbie; 10th June 2010 at 06:29 PM.
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Old 10th June 2010, 06:53 PM   #19
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The grounds of the OPT secondaries (and therefore the speaker jacks) and the input jacks are all connected together. If you isolate you input RCAs from the chassis, then all of those grounds are floating relative to the earth ground. They key point he is trying to make is that the PCB should only be grounded to earth from ONE location and that location should be the same location that earth ground attaches to the chassis. This is called a "star" grounding scheme, though it's not much of a star because the PCB collects a lot of the grounds for you.

Here he suggests using one of the RCA jack grounds. This is convenient especially if your RCA jack are NOT isolated from the chassis. In that case, that RCA jack is also the chassis ground location. By attaching the earth ground there as well, the RCA becomes the star ground location.
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Old 11th June 2010, 02:10 AM   #20
Ian444 is offline Ian444  Australia
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Thanks Russ. My amp runs nearly 40mA per tube, about 175mA all up, this would be around 4.6W power dissipation. Not to worry.

Ian.
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