Experiments of the DAC kind

[NRG]

The pre-tracking reg cct is on page 20 of the Nat Semi data sheet for the LM317. You need about 5v of headroom IE 12v out means min of 17v in.

The cct omits the set resistor bypass caps and o/p caps but just add those in. Remote sense of the load is detailed on p9 under "Load Regulation", essentially take the earth connection of the set resitor and bypass cap to the load.

From what I've learnt by first hand experience and also read to get the best from these devices they need to supply something like 30mA minimum (more is better), make the o/p cap a good quality 'lytic but not one that is low ESR, I use Starget ROD here, value 100~220uF. The set resistor bypass cap can be low ESR and value 47~100uF but I also use the Stargets here. Heatsink them well.

[freddiecas]

thanks, I'm confused about where the second set of leds go, the first replaces R4 in the page 20 diagram? does the second replace the R2 720 ohm?
and then can you tweak the output voltage with the 120 R3?
cheers.

[NRG]

Ah! my pictures probably caused the confusion.

The LED's are used with two single regulator set for 5v operation feeding the analogue and digital side of the CS8412 but the same principle would apply if it was a pretracking configuration. The diodes replace the normal set resistor (R4 in the pretracking diag). Voltage drop on a std green LED is 1.9v or thereabouts, two in series makes it 3.8v plus the 317 1.25 ref voltage gets you to 5.05v more or less. Lower noise, more accurate voltage setting and lower o/p impedance.

The pretracking reg I pictured is set for -15v, it does not use LED's...for this I used a 13v Zener and 1N4001 diode to set o/p voltage. 13+0.6+1.25 = ~ 14.85v The Zener again replaces R4 in the NatSemi diagram.

Looking at it again I have set R1=R2 (240R is fine), bypass R1, bypass R4 or Zener if you want to try it and place a cap on the o/p of the second 317.

Take the earth for R4/Zener and bypass cap to the load, don't connect to the same earth as C2.

--Neal

[freddiecas]

ah! I see. I've seen led references for the noise performance before so was intrigued by the pics

sorry to bypass your thread away from your dac mods Neal

It looks like I'll have to buy another 317/337 and maybe up the voltage on the torroid secondary. A 15-0-15 at 30VA should have given me 4V across a single 317/337 for +/-15V out. Might not be enough for the double version because approaching dropout?

18 x OPA134 at quiescent of 5mA each in a relatively high impedance active crossover should be less than 200mA for the supply I'm guessing.

thanks again, you've helped a lot to get a really good supply design, I had not considered the pre-tracking idea. My "ground plane" will be a thick copper wire across the vero ground outputs of 6 parallel sallen key circuits, so I can sense the ground from the centre point there.

cheers, Fred

[NRG]

No problem Fred, 15-0-15 may just be enough.

Full bridge rec would yield 21.2v, let’s say 20v with 10% regulation. You need 5v head room for the pretracking cct.

But you also need to consider the ripple voltage, so a pretty large cap would be needed as ripple voltage always reduces the Vdc by half of the ripple. At 50Hz and 200mA a nice big 4700uF cap on the bridge o/p would reduce Vdc by about 0.2v.

Your current draw may be a little higher as there is also a constant current flowing through each ‘program resistor’ between the o/p and Vadj, if you go for 240R then you will have 5.2mA flowing for each regulator in addition to the cct draw. So maybe another 22mA on top.

Also take into account ripple current, although not large in this case, as being 4~6 times larger than Adc at approx 1.2A, rate the diodes and ripple capacity of the cap above this to be safe.

Edit. If you look at the dropout V chart on page 6 you can see at 200mA and a case temp between 25 and 50C the droput V is about ~1.6 so I think you will be OK.

Also forgot you will drop about 1.2v across the bridge rec diodes so maybe cutting it a bit close with standard diodes but as its for audio put in some nice Schottky diodes to regain the dropped volt and improve the sound quality

--Neal

[freddiecas]

thank you Sir! , the circuit I was sort of basing it on was for a Kef Kube from the 80s.

This had 2R 2W resistors from the sec to bridge, then 1000uF after the bridge, then another 2R 2W then another 1000uF then 4.7uF around the input of the 317/337 then more 4.7uF around the reference and output.

I had upped the 1000uF to 2x 2200uF keeping the RC filtering, and 3x10uF plus 3x0.1 ceramic around each of the regs. Then each parallel leg of the actual circuit board would have 10uF (total 60uF) with 36x0.1 ceramic around each individual op-amp supply. Or so I was planning

Cheers. Fred

[freddiecas]

if I use a 13V zener ref to get 14.25V out (giving a little extra safety across the regs) is 240R also ok for R3 (3 x 240 in all)?

and how do I put cap discharge protection diodes now, one across R3, one across R1 and one from output of 2nd reg to input of first?

thanks

[NRG]

Hi Fred,

RC filtering is a good idea but probably unnecessary with the pretracking configuration for ripple. Also can you afford to lose .4v across each 2R resistor?

The PSRR of the pretracking cct is very good, the benefit of an RC cct is probably the low pass filter it would form… 2R and 2200uf creates a LPF at 36Hz…maybe worth forming a 2200uf = 2R = 2200uf PI filter after the bridge….if you have enough volts….

I experimented with resistors in the secondary when messing about with gainclone amps and found they slowed down the bass and transients…although the current demands where much greater than your cct, I dunno I would tend to leave them out and use soft switching Schottky diodes.

I loathe ceramic caps, they suffer from Piezo electric effects, the capacitance varies with applied voltage and they can inject noise into the cct. I know some are better than others in these respects but I would avoid them if it where me.

The input to the 317 would only need a small decoup cap if they are positioned some way away from the supply, otherwise no need. I would decouple the supply at each opamp with a 0.1uF Wima cap and also place one directly across the – and + supply right under each opamp keeping the leads short as possible.

240R for R3 is fine. I’ve not used protection diodes but it may be prudent to place one from output to input on each reg if you are planning to use a large cap on the output, see Figure 4. I would not bother with the Adj pin diode.

If going for the Zener you can raise the output voltage by .6v with a diode to ground. Ff you use two Zeners in parallel you will halve their impedance. There’s a good info about his on Acoustic.org on the first page about 3 terminal regulators.

--Neal

[freddiecas]

thanks again for your stirling advice I apologise for not mentioning that the torroid was planned to be remote in a plastic box and 1 meter of 3 core cable would take the secondaries to the circuit box containing bridge and rectifiers to give you the full picture. The primary will end up connected all the time with the secondaries switched through a 3 pole for power.

Ok I'll ditch the ceramics for polyester or polyprop, and ditch the secondary 2 ohms, keep the 2 ohms between the two 2200uF and up the torroid to 18-0-18 to be safe. That would be around 9-10V across the regs for a 13V zener, say 5V max on each so around 1W so would one of those small U section heatsinks with fins down each side for upright TO220s be ok?

[NRG]

Yes, Fred should be OK...you'll soon find out though, if they are painfully hot when you touch them then you'll need larger ones!