[Sean H]

Beautiful, absolutely stunning! Nice job. Welcome!

 

[pabbi1]

Welcome, and very nice work.

You will find a small, but dedicated, core of SDS devotees here in the DIY, thanks, of course, to Sheldon.

 

[amb]

Thanks again everyone for your welcome. This is a great place.

pabbi1, I noticed in your profile you say I'm scared... 2.7k resistors on
R11/R61 for the SDS. LOL. That means you're running a voltage gain
of 3.7. I guess I should be *really* scared then... I am running a
gain of 5!

I thought I'd post a rundown on how my SDS amp differs from Sheldon's
original v1.2 design, as follows:

• I eliminated R12/R62, and substituted R10/R60 with 4.7mA CRDs
  (Vishay/Siliconix CR470). This sources a steady 4.7mA current out of the op
  amp output for class A operation (rather than sinking an amount of current
  that varies depending on the bias pot setting and supply voltage). Also this
  greatly improves the PSRR.
• Added a 475 ohm resistor between the volume pot's center contact
  and the input of the amp. This is so that when the volume is all
  the way down, the + input to the op amp is grounded through that
  resistor rather than directly. This balances the 475 ohm resistor
  on the - input side (see 4. below). Not a big deal with the FET
  op amps however.
• I installed R5/R55 (1M ohm) anyway, just in case the volume pot
  should go open circuit or become disconnected, the + input of the op amp
  won't go floating which would be bad.
• The feedback resistors I used are R11/R61 = 1.82K, R6/R56 = 475 ohm
  for a voltage gain of 5 (actually, 4.83 to be exact).
• I used a 33pF capacitor for C16/C66 to ensure stability with
  the ultra fast op amps. This adds a roll-off pole beginning at
  2.6MHz toward unity gain. However I reduced C11/C61 to 10pF. This
  gives optimal step response with the AD8066AR while also working fine with
  the other op amps I tested with.
• I replaced R2/R4 with 10K 25-turn Bourns cermet trimpots to allow me
  to adjust the regulator voltage. I have it set to ±12V to work
  with the AD8066AR and AD8620AR. I found that even with hand-matched
  fixed resistors, the positive and negative regulators do not match
  each other in output voltage very well, so using the trimpot allows
  me to fine tune it.
• Added a 4700uF 50V Elna electrolytic cap each in parallel with
  C5/C6, these are the large chassis mounted caps at the rear of the amp.
• Added extra bypass capacitors to the power supply: 0.47uF tantalums to the
  input side of the voltage regulators, 0.22uF to the output side.
  In addition, I added a 0.1uF multilayer ceramic cap directly across the
  V+ and V- pads of the op amp.
• I used 121 ohms for R7/R57 because I happen to have these around.
• R13/R14/R63/R64 are 332 ohms (MOSFET gate resistors).
• R8/R9/R58/R59 are 2W flameproof metal film resistors.
• The Q1/Q2/Q51/Q52 MOSFETs are IRFZ24N (N-ch) and IRF9Z34N (P-ch). I
  have not seen anyone use these devices in this forum, but they work
  great and are available from Digikey and Newark.
• Q3/Q53 are Motorola 2N5210 low noise NPNs that I have some of.
• The op amps are AD8620AR, AD8066AR or OPA2132PA (socketed for easy swap).
  The AD devices are mounted on SOIC-to-DIP adapters.
• The bridge rectifier is an IRF 4GBU06.
• I used 1N4004 instead of 1N4007 for D1/D2 because I have some around.
• C1/C2/C3/C4 snubbers are 330pF multilayer ceramic caps.
• I used a Noble 50K stereo pot instead of the 100K Radio Shack unit for
  the volume control.
• Different brand of resistors and capacitors, etc., than what Sheldon
  specified in his parts list. E.g., Vishay/Dale RN55 1/8W resistors
  instead of the Yageos. Nichicon, Kemet, Vishay, and Xicon caps, etc.
• Added a 3mm blue 3.5V LED with a 22K resistor in series, driven
  across the unregulated supply rails (+22.5V to -22.5V) as a
  power-on indicator.

In my case the extra power supply bypass caps were essential. Without them,
I observed a 1.6MHz oscillation from the voltage regulators (seen with
my Fluke scopemeter on both rails, but higher amplitude at around 200mV p-p
on the negative side). The oscillation occurs even without the op amp
plugged in, and feeds right through to the output.
This of course is not audible in itself, but I found some side effects.
When the MOSFET quiescent current was increased to over 50mA or so, there
was a slight hissing noise heard through my HD600s. Also, when powering up,
there was an annoying bbrrraaaap noise for about a half second.
These are all completely eliminated with the added bypass caps. Now
the regulators run rock steady.

I think this speak volumes about the importance of an oscilloscope
when building an amp of any type. Otherwise the symptoms may have been
ignored or dismissed as something else, but surely would have adversely
affected the performance of the amp.

Anyway, sorry about the long post. I hope this info is interesting or
useful to SDS amp fans, or those who are looking to build an SDS amp.

-Ti

 

[IMEP]

Congratulations! Very nice work.

 

[pabbi1]

That is impressive... did you make all these mods based on your experience, or in consultation with Sheldon? It would be WAY cool for you to post an addendum to Headwize with your modded parts list, with your post as to what was modded and why.

Yes, I was considering a higher gain, but feared extra distortion in trade. That said, I may yet bump R11/61 a bit more... and, I may yet replace the Blue Velvet with a Vishay-Dale stepped attenuator, but I may save that for a version 1.2, since I built a version 1.1, board and parts.

 

[amb]

Quote:

Originally posted by pabbi1 That is impressive... did you make all these mods based on your experience, or in consultation with Sheldon?

I made the changes on my own without consultation with Sheldon. I did send Sheldon an email after I completed the amp to let him know about these changes, though.

Some changes I made were based on what parts I had on hand. There is nothing magical about those. They should have little or no impact on the quality or performance.

Other substitutions are based on various reasons. For example, upgrade to higher grade parts (op amps, resistors, caps, pot), or replace parts that are out of production (MOSFETs), or possible improvements (adding more capacitance to the power supply filters), or fixing a problem I encountered (oscillation issues), etc.
I think I listed the reasons for most of these.

Quote:

... but I may save that for a version 1.2, since I built a version 1.1, board and parts.

BTW, in case you're not aware, Sheldon's v1.1 board had a layout bug, and is corrected in v1.2. Two of the four snubber capacitors around the bridge rectifier were hooked up wrong. See this post for details:

http://www5.head-fi.org/forums/showt...threadid=29081

HTH.

-Ti

 

[amb]

Moderator please delete this.