Go Back   Home > Forums > Blogs > RJM Audio Blog

If I put my notes here, I might be able to find them again later!
Rate this Entry

Sapphire 4 headphone amplifier [Rev. 4.1m]

Posted 24th April 2017 at 01:02 AM by rjm
Updated 21st June 2017 at 09:17 AM by rjm

Development thread starts here.

While I consider this my best work to date, the documentation trail has become hopelessly scattered so let me try to regroup in this blog post and make some sort of gateway/portal to the project. [Update: web page write up now finished.]

The followup to the Sapphire 3 started out as a generic musing on stacked diamond buffers and current mirror amplification, coalesced for a bit as the ill-fated Project Unity before becoming part of the Sapphire line initially as a temporary measure as something I could drop in my existing chassis for testing. The first iteration kept the open loop buffer of previous Sapphire iterations, but now I'm running the 4.1m boards with the buffer inside the feedback loop I realize I've ended up at just the classic, dictionary definition current feedback amplifier (CFA). Sigh. So much for originality. It sounds great though, so I'm not as upset about it as I might have been.

*****

The output stage buffer is left outside the feedback loop when R3 is populated in the schematic below, or included within it with R3X populated instead of R3. In both cases feedback is applied to the current input terminal (emitters of Q3,4), so the voltage amplifier is always running closed loop. The difference is in whether the feedback network R3/3X and R2A,B,C is driven by the low impedance buffer output or the high impedance current mirrors Q7,8. This makes a very big difference in how the amplifier performs. To be clear: We put the buffer inside the feedback loop not to improve the performance of the buffer but to improve the performance of the current feedback amplifier.

Performance varies with gain, configuration, and loading. Closed loop: Simulated harmonic distortion is about -100 dB at 0 dB fundamental for 60 Ohm load, 27 dB gain. Measured THD+N unloaded for 25 dB gain is -94 dB A-weighted, with a noise level of -102 dB A-weighted and -95 dB crosstalk (RMAA). I hesitate to give an output power since I haven't measured it, but it's going to be a lot... simulated, into 60 ohms, in excess of 100 mW before distortion breaks -80 dB. Figure minimum 25 mW over 15-600 ohms loads.

Second harmonic is considerably higher open loop, especially at high gains. -82 THD + Noise, dB (A) for 27 dB unloaded, measured (RMAA). This is essentially independent of load as the nonlinearities are generated in the current feedback amplifier not the buffer.

*****

The 4.1m boards feature the option of closed loop / open loop operation, as well as a 3 position gain jumper switch. Further details can be found in the attached BOM.

I run mine closed loop at 25 dB gain with HD-600 headphones.

As noted in the BOM the boards can also be configured as a line stage / preamplifier.

*****

41m3 BOM changes the recommended resistors in the R3X configuration to increase the phase margin a little. R3X 1k was perhaps cutting it a bit fine.

*****

After seeing through quite a few Sapphire 4s built by myself and various others, the basic design and layout appears sound. Everyone has been quite pleased with the results. The only residual issue is the DC offset. DC offset is controlled by the trimmer R6, but the available control range is defined by hfe missmatch of Q1, Q2 and also the missmatch of the Zener references voltage of D1, D2.

Basically the situation is this: if you match the Zeners in advance you are guaranteed not to have problems, if you leave it to chance and draw the absolute worst outliers it's possible that the trimmer is out of range to zero the output. It hasn't happened so far with R6 = 5k but I concede that it's a possible pain point.

Typically you'll find the hfe of Q1 and Q2 are about 60 units apart, but up to about 100 difference in hfe should not present any problem at all and there is no need to match transistors. The Zeners on the other hand sometimes come in at several hundred millivolts different. Ideally you'd prefer to use pairs which are less than 200 mV apart.

Once set the offset remains zeroed and does not drift much with temperature. 50 mV or less seems to be typical as the unit warms up.
Attached Thumbnails
Click image for larger version

Name:	pcb-sapphire-41m-brd.png
Views:	146
Size:	20.9 KB
ID:	2360   Click image for larger version

Name:	sapphire-41m.png
Views:	104
Size:	198.3 KB
ID:	2361   Click image for larger version

Name:	pcb-sapphire-41m-sch.png
Views:	262
Size:	20.4 KB
ID:	2362   Click image for larger version

Name:	DSC_1165s.jpg
Views:	177
Size:	203.3 KB
ID:	2363   Click image for larger version

Name:	DSC_1172s.jpg
Views:	137
Size:	386.5 KB
ID:	2364  

Click image for larger version

Name:	sapphire-41-ltspice R3X 15dB.png
Views:	132
Size:	83.5 KB
ID:	2376   Click image for larger version

Name:	summary.png
Views:	79
Size:	21.8 KB
ID:	2377  
Attached Files
File Type: zip pcb-sapphire-41m3-bom.zip (195.9 KB, 66 views)
File Type: asc Sapphire 41.asc (6.2 KB, 114 views)
Total Comments 3

Comments

  1. Old Comment
    Looks like a nice amp! I am always curious about parts selection, so is there a reason for choosing BC327/337 over BC550/560 or BC547/557? It looks like they may be a little more robust than the 547/557. Are they a type of "double-die" part?
    permalink
    Posted 24th April 2017 at 12:41 PM by needtubes needtubes is offline
    Updated 24th April 2017 at 01:10 PM by needtubes
  2. Old Comment
    rjm's Avatar
    I started by tabling up all possible devices, then shortlisted to those which were commonly available and further limited the list to transistors where the datasheet included audio as an application. High voltage types not needed so those were dropped. I'm pretty sure I ended up with a final selection which included the BC327/337, BC550/560, and BC547/557 and as you know there is not much to choose between them. Someone here at diyaudio recommended the 327/337 to me as being a good sounding transistor ... so I went with that!
    permalink
    Posted 24th April 2017 at 11:39 PM by rjm rjm is offline
    Updated 24th April 2017 at 11:42 PM by rjm
  3. Old Comment
    Fair enough! I have found BC550/560 to sound nice, so may add some BC327/337 to my next order to give them a try as well.
    permalink
    Posted 25th April 2017 at 01:09 AM by needtubes needtubes is offline
 

New To Site? Need Help?
Copyright 1999-2017 diyAudio