Upgrading the Acurus Pre

mondialfan

Journeyman
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#1
Last month Lee contacted me and inquired about upgrading an Acurus LS11 that he had. Because there are others on the forums here that use the Acurus pre's we thought that we'd make a thread out of it for others to watch and ask questions along the way. Lee's LS11 arrived yesterday so away we go... This LS11 is a very late production unit. This is evidenced by the black lower chassis and black back panel. These only came out from what I can tell in the last two years or so of production. One of the primary storage caps has a 15th week of '98 date code on it so this pre is at the latest a late '98 model but more likely a '99 or '00 model. I don't think that these were built much past the year 2000 after the Klipsch buy out.

For this work the entire power supply will be upgraded. The 5VA transformer is swapped for a 10VA, the standard 1 amp rectifier diodes are replaced with 2A soft recovery types. The 3,300uF primary storage caps are replaced with 8,200uF Nichicon's and the regulated DC rails receive new e-caps as well. The two primary storage caps will be bypassed with polypro film caps. A few of the incoming audio path resistors are replaced with Dale low noise metal films. In addition I had some boards built to house an LED biased current source for the front end complementary diff inputs. These small boards are designed to sit in the same locations as the LTP tail resistors. I typically see a modest reduction in THD+N across the 10Hz - 20kHz band with the addition of the active current sources.

Upon receiving a piece of gear for upgrade I will usually run a few tests on it to see if any glaring problems exist. As luck would have it this Acurus pre is one of the few I've ever seen that actually had a problem. Not a major problem mind you as its still passing the OEM THD+N spec, but it's not proper. I included a couple of charts from the AP to show what's going on with the pre. The first chart shows THD+N vs V-out. The left channel is basically a textbook curve for one of these Acurus pre's. Its what I expect to see, +/- a few dB of that line anyway. The right channel has issues and is showing about +5dB worse noise at higher voltages.

Moving on to the second chart (THD+N vs Freq) shows the same issue exists at higher frequencies. Once you drop below ~200Hz things are starting to look more normal for the right channel.

So I will need to correct this issue before moving on to the upgrade work.

I began by performing a the following checks on the preamp to isolate the problem. Voltage checks are good, resistances also look fine. DC offset is great, like 1mV in the problem channel and 20mV in the left channel, this is actually better than most Acurus pre's I've measured. Residual noise showed no issues as both channels were approximately 16.5uV. To eliminate the front end switches and potentiometers as a source of the noise I connected the AP's generator output directly to R4B which is the signal input resistor for the right channel just before the complementary diff inputs. Running a distortion test in this manner resulted in a curve nearly identical to the first THD vs V-out chart, so the switches and pots are not a problem.

I used up my freeze spray a few weeks back and haven't replaced it yet so I decided to try the opposite and use some heat to see if anything jumped out at me. I set my Weller solder station down to 300F and started probing while watching the distortion of the right channel on the AP. When I got to the inverting side of the Comp diff inputs both the PNP and NPN transistor showed a slight drop in the THD+N of 1 - 2dB when they were heated. A bit more probing and the source of the distortion popped up, the 10pF mica capacitor in the feedback network. Once I hit that with the heat and then moved the iron away from the mica cap the distortion would drop right down to match the Left channel. I then did the same thing again and ran a test. You can see that in the third chart the Right channel mostly follows the left. The distortion jumped back up a couple of times, but this Mica cap should be whats causing the elevated distortion issue. I have some 15pF caps so I may install one of them temporarily to test the issue until I can get some new 10pF caps in from Mouser.

Once this problem is out of the way and the preamp tests good I'll move on to the upgrade work.

THD_vs_V-out_100kOhms.jpg

THD_vs_Freq_4Vout.jpg

THD_vs_V-out_10pFcap.jpg
 

Northwinds

Veteran and General Yakker
Joined
Mar 18, 2013
Messages
7,325
Location
Coventry, CT
Tagline
Fondler errrr... fan of all Nav's avatars
#3
I almost got a 11 a few weeks ago locally but could not justify a third preamp and did not want to give up a TT for it, My Perreaux and Apt Holman are totally up to snuff but I may get one someday just to see how it sounds with a big amp
 

mondialfan

Journeyman
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#4
Northwinds, I have 5 pre's here in the house now so I understand what you mean by not needing another. Unfortunately I keep looking for deals though. I think its an illness at this point. o_O I had an RL-11 show up locally on CL a few months back, mint condition, original owner, included remote and box. Did I need it? NO! But at $200 I couldn't have driven to the North side of Dallas to get it any faster and not wound up in jail. Sometimes there's just no way to resist the deal... A few weeks later a very clean LS-11 showed up on CL for slightly less. I barely managed to talk myself down from buying that one by using the killer deal on the RL-11 to justify walking away.
 

mondialfan

Journeyman
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#5
I stripped down the LS11 this morning and swapped out the bad 10 pF mica cap with the new 15pF that I had in my parts bin. I included a picture of the nude LS11 in my testing cubby hole. I used to do most of this stuff on a workbench in my garage but its miserable out there in the summer. Now I only solder, perform initial adjustments and variac/dim bulb testing out there.

I included some new charts showing the THD+N curves after replacing the mica cap. Now the curves follow each other nicely with only a few dB difference, which is exactly what we want to see. Now the LS11 is ready to continue on with the upgrade work.

The additional capacitance in the feedback circuit shifts the Freq response down slightly. Looking at it with the AP with 10pF in this location at 200kHz the stock left channel is at -0.5dB and the right channel is -0.8dB @ 200kHz. Stock freq response in this pre is approx. -3dB @ 250kHz. Due to the fact the AP only goes to 200kHz I have to use a function generator and HP400F that I have to measure the Acurus and Aragon preamps out to -3dB. Typically I forego this and just look for the mild drop at 200kHz.


Testing.jpg

THD_vs_V-out_10pF_Replaced.jpg

THD_vs_Freq_4Vout_10pF_replaced.jpg
 

mondialfan

Journeyman
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#8
And the test cart... :)

I bought the wire cart a couple of years back at a local hardware store that was closing. I never realized just how handy it would turn out to be. I know it doesn't look like it but it's amazingly sturdy.
 

laatsch55

Administrator,
Staff member
Joined
Jan 14, 2011
Messages
56,698
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#9
Kevin, does the Acurus have a D/A converter in it?? As in what's the D/A input for?
 

mondialfan

Journeyman
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#10
Nah the D/A input is just another analog input, same as the CD, Phono, etc. The Acurus is a very basic no frills preamp. What they did do well was including good quality parts and design into such a low cost package. The upgrades I perform are to correct the shortcomings that were done to cut costs and hobble performance.
 

laatsch55

Administrator,
Staff member
Joined
Jan 14, 2011
Messages
56,698
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#12
And the test cart... :)

I bought the wire cart a couple of years back at a local hardware store that was closing. I never realized just how handy it would turn out to be. I know it doesn't look like it but it's amazingly sturdy.
I know just what ya mean...
 

Attachments

Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#19
Finishing up with the upgrade work.

First picture is of the unit with most parts to be replaced removed from the board. The old and new transformers are sitting on the left.

Interior1.jpg

Next is the completed unit, current sources are there but hard to see.

Upgrd_Compl.jpg

This is a closeup of the two channels showing the left with the current source boards installed and the right channel with the resistors removed in preparation for the current source boards.

Channel_W_CS.jpg

Next up is the rear of the board showing the bypass caps installed across the large primary storage caps near the rectifiers. The wire added to the one primary cap is to ensure connection between the two points. This is a weak area of the board as the current passes from the bottom to the top though that via in the board. Probably half the people that perform these upgrades open that via up and the preamp will not work afterward. Even if it measures good I install the jumper just for piece of mind.

Rear of MB.jpg

Lastly is the cleanup, this is the perfect time to soak the faceplate and knobs in hot soapy water to get them looking like new again. Make sure you blow the set screw holes in the knobs out real good with compressed air to keep them from rusting up.

Cleanup.jpg
 
Joined
Apr 2, 2017
Messages
70
Location
Grand Prairie, Tx
Tagline
---
#20
So you're probably wondering what the performance of the unit looks like after the upgrade work, so here you go.

In the first two charts the white lines are the copied traces from the previous (pre upgrade) graphs I posted. Unfortunately the APWIN software doesn't let me color those lines, only copy them and paste them as white. Those scans were run right after I replaced the bad mica cap in the right channel and are in post # 5 above. As you can see the THD+N vs Volts out has improved by 3 - 5 dB below 6V RMS output. Distortion above 8V is ever so slightly worse and clipping comes on slightly quicker. The good news is that the all important sub 1v region which is where most listening will occur is a very healthy 4 - 5dB improved over the stock unit.

In the next chart showing THD vs Freq you can see the improvement holds out regardless of frequency. Once again the white lines shown in the chart are data that was pulled rom the previous chart that I posted in #5 above.

Also both the right and Left channels are tracking almost on top of each other now, where there was a 1db or more spread between them prior in post #5.

In case you're wondering the test conditions were the same, no monkeying around with filters or changing V-out to make things look better.

THD_vs_V-out_Full_Upgrd.jpg
THD_vs_Freq_4Vout_Full_Upgrd.jpg
X-Talk.jpg
Freq_Resp.jpg
 
Top