goldenreference low voltage power supply

[gepardcv]

For a Dynahi powered by a pair of GRLVs set for 30V output, what transformer specs should I go for? I was thinking 70VCT or 2x35VAC — is that too much?

[kevin gilmore]

i'm thinking of 2 x 30V  but you also might get away with 2 x 28V which I have used in the past, depends on the transformer manufacturer

[Pars]

Stupid question time (don't slap me!).

For a dual secondary transformer, could you wire the secondaries together in phase (1 end of one secondary to the opposite phase end of the other secondary), and use that as the "center tap"?

Va

0a

Vb

0b

Tying 0a to Vb for the CT...

[luvdunhill]

Yup.

[gepardcv]

My turn to ask a stupid question; I'm still learning about transformers.

How does the math for calculating rectified DC voltage work? I assumed that for a 2x22VAC transformer outputting +/-20VDC, the voltage drop is (2*22-2*20)=4V (is that right?). Some of that goes into the bridge. In that case, how can a 2x20VAC transformer end up with a +/-20VDC output? The voltage drop then comes out to 0V, with nothing left over for the bridge.

[kevin gilmore]

general rule of thumb is that peak unregulated capacitor value is 1.4 x rms output of transformer for a full wave bridge.

so 22*1.4== about 30 volts unreg input the regulator needs 4 volts, so you still have 6 volts of overhead to compensate for low line etc.

issues with how stiff the transformer are (how much it sags under load) lower the voltage a bit.

[sbelyo]

is 3oz copper required on the PCB or will 2oz be sufficient?

[gepardcv]

I'm sure 2oz copper will be fine.

[FallenAngel]

Noob style question - how is the latest board transformer wired? I'm using a dual secondary transformer, so I'm assuming I wire the two AC lines and leave the center GND open. Also, what's the 6th input to the side with resistor to ground for used for?

[congo5]

yes that's right, and use two diode bridges. 

the 6th is if you want to ground the board to case through the 10R resistor next to it.

[FallenAngel]

On 9/25/2016 at 8:40 PM, congo5 said:

yes that's right, and use two diode bridges. 

the 6th is if you want to ground the board to case through the 10R resistor next to it.

Damn, I already populated all 3 bridges. Should I remove the middle one or just leave it there?

[Pars]

Remove it I would think... would have to check the schematic

[congo5]

I agree remove it...............

just cut the legs if you don't want to desolder

google

Dim-bulb Tester

a great thing to have.... and very cheap to make

or variac ...  I use both

the boards I have are goldenreference6 (file name) and says version .44 on board

Edited September 29, 2016 by congo5

[FallenAngel]

Thanks Pars and congo5.

I checked the bottom of the board and it looks like one part of each secondary gets rectified and fed into the positive and negative rails.

I'll just desolder it. Worst case scenario of lifting the PCB trace still has no effect.

Glad I double checked, thanks guys.

Is there any easy table for output voltage resistor selection? Maybe I'm too used to the other forum.

[congo5]

what voltage do you want?

I do not understand Kevin's method 

so came up with this:

desired voltage x 150 -1500 = R8, R9
40v is 40 x150=6000 -1500=4500
30v is 3kR
25v is 2250R

[ang728]

3 hours ago, congo5 said:

what voltage do you want?

I do not understand Kevin's method 

so came up with this:

desired voltage x 150 -1500 = R8, R9
40v is 40 x150=6000 -1500=4500
30v is 3kR
25v is 2250R

Post it again for reminder
for V+: (R8 + R7) / R7 x 10 (reference voltage of D5 - LT1021-10)

for V-: (R9 + R10) / R10 x 10 (reference voltage of D7 - LT1021-10)

congo5's way is also right assuming that you have fixed R7,R10 's value.

Edited September 29, 2016 by ang728

[Pars]

Kevin's formula is more accurately shown as ((R8 + R7) / R7) x 10

[FallenAngel]

On 10/1/2016 at 0:16 AM, Pars said:

Kevin's formula is more accurately shown as ((R8 + R7) / R7) x 10

 

On 9/28/2016 at 11:46 PM, congo5 said:

what voltage do you want?

I do not understand Kevin's method 

so came up with this:

desired voltage x 150 -1500 = R8, R9
40v is 40 x150=6000 -1500=4500
30v is 3kR
25v is 2250R

On 9/29/2016 at 3:26 AM, ang728 said:

Post it again for reminder
for V+: (R8 + R7) / R7 x 10 (reference voltage of D5 - LT1021-10)

for V-: (R9 + R10) / R10 x 10 (reference voltage of D7 - LT1021-10)

congo5's way is also right assuming that you have fixed R7,R10 's value.

 

Thanks for your posts guys. My question was a leading one in an attempt to have simple and direct parts list for builders that may be very skilled in building equipment but may not have a high level of understanding of schematics or circuits.

I was hoping someone would come up with a little table like the following.

((R8 + R7) / R7) x 10
((1.5 + 1.5) / 1.5) * 10 = 20v
((2.21 + 1.5) / 1.5) * 10 = 24.7v
((3 + 1.5) / 1.5) * 10 = 30v

R7 = 1K5
R8 = 1K5 for 20V
R8 = 2k21 for 24.7V
R8 = 3K for 30v

R9 = R7
R10 = R8

[luvdunhill]

How many years have you been building and selling Audio projects?

[Pars]

Just an fyi, but the Excel BOM that I have has 25V tantalum caps, while Kevin stated 35V in this post:

Note the part number provided above is actually a 20V part. Correct part numbers would be something like 581-TAP476K035CCS and 581-TAP106M035CCS for the 47/10uf parts respectively.

Sorry, just getting an order together for the remaining parts to finish one of mine (v.42 or goldenreference4) boards. I haven't done a spice model or analysis to see what you really might need here, and I assume a number of these have been built with no problem according to the spreadsheet I had (which I can't find here on HC anymore).

 

[luvdunhill]

I can do the math for you, which part?

[Pars]

C7, C8, C9, C13 for the 47uf

C3, C16 for the 10uf

Assume a supply setup for +/-20V output, 2x23VAC input

[congo5]

I used 581-TAP476M020CCS on six or so units

and 35v for the 10uf 581-TAP106K035SCS

from the semantic it looks like they only see 10v

C8 & 9 are fed by the lt1021-10

C7 & C13 are in parallel with a 9.1v zener

and C16 & C3 go rail to the voltage divider/ inverting input, which is also 10v

I remember wondering about that 20v part too.

so far no problems with them, using 20v to 40v output

[Pars]

Yes, looking at the schematic, I would agree that even 20V would probably be OK. I'm using 35V for the 47uf parts in mine, and 25V for the 10uf currently, but that is just what I either had or sourced.

[luvdunhill]

The 10uF has a startup transient up to around Vout. Probably okay in reality but settles down to Vout-10. The 47uF behave well, and zener and reference voltages should be fine there.

I am using 16V on both 47uF and output rail on the rest. You can use much smaller than the 10uF called for and be fine, so don't be too concerned about that value.