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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

AKSA's Lender Preamp with 40Vpp Output
AKSA's Lender Preamp with 40Vpp Output
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Old 25th November 2018, 12:11 AM   #901
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
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Now with Mu-follower output stage that allows driving low 600ohm loads:
Click the image to open in full size.
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Old 25th November 2018, 11:12 AM   #902
marconi118 is offline marconi118  Germany
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Is it a gyrator?
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Old 25th November 2018, 11:34 AM   #903
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
No, just Aksa Lender in dual rail and built in CRCRC, and now with mu-follower output.

Click the image to open in full size.
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Old 27th November 2018, 10:49 AM   #904
Skylar88 is offline Skylar88  South Africa
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This transformer I've used seems to be real crappy and inefficient. The laminations are 0.5mm thick, thus eddy current loss must be high.The windings are probably very thin too, resulting in excessive IēR loss as well.

Click the image to open in full size.


My AKSA Lender pre chassis is not ventilated, and it runs as hot as an ACA. I've sent the lid in for laser cut ventilation slots, but maybe I should get rid of the heat source anyway.

I'm considering to rather us an internal SMPS, like in the Sutherland N1 preamp pictured below. My question is: (for sizing the SMPS) What would the current draw be on the AKSA Lender (15dB version)? ...and if I measure it, would the current draw vary with sound volume? i.e. should I measure at high level?

Does that look like a Cap Mx on the N1 Sutherland PSU board?
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Last edited by Skylar88; 27th November 2018 at 10:52 AM.
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Old 27th November 2018, 11:29 AM   #905
Puggie is offline Puggie  United Kingdom
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That looks like a dinky little meanwell supply in the Sutherland.

very tidy!.

EDIT: Most of the little meanwells are available as 48Vdc output, would it be sensible to run one of these straight from the mains to 48v then your CRCRC filter to the pre-amp?

Last edited by Puggie; 27th November 2018 at 11:31 AM.
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Old 27th November 2018, 12:11 PM   #906
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Hi Skylar,
The Aksa Lender Preamp runs at about 15mA to 17mA bias current in stock form. So dissipation across the DN2540 and 2SA1837 equals 48v x 0.017A x 2 ch = 1.6w. The cap Mx is 4v x 0.017A x 2 = 0.14w. The DC-DC step up is about 90% efficient and assuming it uses a MOSFET with 4v drop, again another 0.14w. So about 2w total. Your little transformer may be adding a couple more watts. Maybe 4w total which can get warm if not heatsink or vented. Some air slots at the top and bottom should help cooking it off, or try using an external wall wart Class 2 transformer. I don’t recommend bypassing the cap multiplier. Besides filtering ripple it gives a slow start ramp up to prevent thump. Meanwells may work - that unit looks like it might be noisy. Also the DC-DC you have is noisier than the white aluminum ones I recommended.

What value source resistor are you using after the DN2540? If you are using lower than 47ohm there your heat output will be significantly higher. Remember that for headphone use, the current is higher but Vcc dropped to 24v. 48v at 75mA would be a lot of heat (7w a pair from DBs alone).

I just ordered the M2X Melbourne DB PCBs. They might make useful standalone preamps and can be DC coupled on output. Just need fairly clean +/-24v supplies.
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Last edited by xrk971; 27th November 2018 at 12:17 PM.
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Old 27th November 2018, 01:52 PM   #907
Skylar88 is offline Skylar88  South Africa
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Quote:
Originally Posted by xrk971 View Post
What value source resistor are you using after the DN2540? If you are using lower than 47ohm there your heat output will be significantly higher. Remember that for headphone use, the current is higher but Vcc dropped to 24v. 48v at 75mA would be a lot of heat (7w a pair from DBs alone).
Thanks X.

R11 = 2K2 for 15dB gain, as per your advice.

I’ll wait for the chassis to come back from the engineering shop and see if the ventilation slots make a difference. If that doesn't solve the heat problem, I’ll try a 15W Meanwell (RS-15-24) that is currently being used for the B1.

I'll also try to get the lower noise white DC-DC converter at some point.
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Old 29th November 2018, 01:14 PM   #908
xrk971 is offline xrk971  United States
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AKSA's Lender Preamp with 40Vpp Output
Hi Skylar,
Any luck with getting the preamp case to run cooler? I was actually asking about the bias current setting resistor R15 (in schematic below) that is at the source of the depletion mode DN2540. Default is 47R which gives about 16mA bias current. On the headphone amp, the Vcc is set much lower at 24v and the bias much higher (about 80mA). Running 80mA at 48v would indeed be very hot - you would burn your finger if you touched the heatsinks on the DN2540 or 2SA1837.

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Old 30th November 2018, 04:56 AM   #909
Skylar88 is offline Skylar88  South Africa
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Quote:
Originally Posted by xrk971 View Post
Hi Skylar,
Any luck with getting the preamp case to run cooler? I was actually asking about the bias current setting resistor R15 (in schematic below) that is at the source of the depletion mode DN2540. Default is 47R which gives about 16mA bias current. On the headphone amp, the Vcc is set much lower at 24v and the bias much higher (about 80mA). Running 80mA at 48v would indeed be very hot - you would burn your finger if you touched the heatsinks on the DN2540 or 2SA1837.
Hi X,

R15 is the 'standard' 47R5 and the heatsinks don't get very hot. Although it's running at 48V, I can keep my finger on the sinks indefinitely. It's the PSU transformer that makes the most heat.

I'm still waiting for the chassis parts to come back from the engineering shop, but I'm hoping the ventilation slots will make a big difference. If it doesn't, I'll shoehorn a 15W Meanwell in there.
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Old 3rd December 2018, 09:02 AM   #910
Puggie is offline Puggie  United Kingdom
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Received my boards thanks XRK, they look great but bigger than anticipated (should have measured them I guess, rather than 'yeah that should fit ok' )

My plan is to run this as a front end to an ACA or possibly MoFo, do you see any issue in pulling 24v from the power amp and just mounting the DC-DC step up in the pre amp case (I don't see a problem). My only other thought would be any suggestion for a nice connector for the DC (I cannot really think beyond the 2.5mm DC barrel plugs used for 24v in on the ACA). I'm really looking to avoid excessive external PSUs.
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