Last Updated April 6th

On March 25th, Line 6 released firmware version 1.22 that is supposed to address this bug and a bunch of others. The general consensus to this update is that it improves the sound quality of  many amp models, but many still feel it is not completely fixed. On April 5th Line 6 announced firmware 1.3 that adds a few new amp models and a bunch of amp controls that may possibly make this problem a thing of the past. This update is supposed to be released by the end of April. So we should know soon.

I have changed the status of this tread to "Answered", even though it is currently not, because this thread has grown extremely long and unwieldy making it impossible  to track all the different discussions. Please start new threads that  address more specific issues so they can be more easily digested. Thanks  to everyone who participated in this epic thread! AZGDude.

I thought I would try to summarize this thread since it has become MONSTER sized with over 50 pages and gone off track many times making it difficult to to find useful information in it.

Here is the original post that stared it all.

AZGdude wrote:

 

I have now had a HD500 for about a month now and I am pretty happy with it overall. But some of the amp models have a very harsh and fizzy top end when strumming chords hard. It is most noticeable to me with the AC30, Blackface Double and Hiway 100, but I also hear it with a few other models. I spent a couple of hours last night trying to get a nice sounding Vox AC30, but was not able to create a sound nearly as good as one of the AC30 presets in Amplitube 3 or from what I remember from playing a real AC30. All the HD AC30 presets I have tried or created always sounded to harsh and fizzy and if I dialed out the harshness I also lost the AC30 chime.

 

Are others finding similar problems? I am wondering if it is a problem with the HD500 or something with my setup. I have an ESP LP copy with Seymour Duncans (JB bridge, 59 neck) and run directly from the HD 1/4 outs to KRK Rokit 6's. I have tried changing "Guitar In" from normal to PAD, but it does not seem to change much. All the same equipment sounds great when playing AC30 presets in Amplitube 3 (except I am using a Alesis i02 Express as the interface between guitar and monitors). Anyone having similar problems or know a solution?

The Problem:

The HD500 (and possibly the HD400 and HD300) produces a distinct digital clipping noise or fizzy higher frequency hash/white/beehive noise when strumming chords hard. It only seems to happen on full amp models that include the preamp and poweramp and is most audible in cleaner amps like the Fender Twin Reverb and Hiwatt. From my experience the HD500 exhibits the fizzy hash/white/beehive noise when it is nearing its dynamic range ceiling, and the digital clipping sound when it surpasses the ceiling.

Everyone does NOT seem to have this problem, and those that do don't have a problem plugged into any other amp or modeler. Different guitars react very differently when plugged into the HD. Since originally posting this question I have given into GAS and bought a new Fender American Deluxe Strat with N3 pickups and S1 switching. I have quickly realized there is a world of difference in the sound quality of the two guitars plugged into the HD500. The Strat sounds incredible with an airy and large dynamic range  and large tonal variances depending on touch, selected pickups, volume and tone  control settings, plus there is little or no sign of digital clipping or hash/white noise. The Strat also seems to work substantially better with overdrives, compressors and other effects on the HD. Conversely, my ESP with Seymour Duncan 59/JB HB pickups sounds very compressed and hashy, like the amp models are pushed to the absolute edge of what they can handle. I am guessing this is how Nigel Tufnel's amps sound at eleven  Not at all the way it sounds plugged into other amps and equipment. Plugged into other amps and equipment the ESP has a warmer sound with stronger bass content which may be part of the problem (see comments below about frequency).

Out of curiosity I crudely checked the voltage output  of both guitars using a cheap voltage meter (as suggest long ago by  Crusty). With the Strat I maxed out at ~16mV with single coils and  ~300mV with the three single coils in series. With the HB guitar I maxed out at ~900mV with the JB in the bridge position. The 59 maxed out at 300mV - 400mV. My understanding is that anything under 1000mV is considered normal for a guitar, but it makes me wonder if  voltage plays any part in this problem?

There are two lines of thought as to the source of the problem. The leading theory is it's a software issue that is brought out by certain guitars or pickups. The problem does not seem to be based strictly on how hot pickups are, because some people with low output Strats have the problem, but more related to specific frequencies. Recently a forum member did some testing and found that the clipping and fizz is more audible in lower frequencies. His full post can be found here. This theory is supported by multiple people exchanging their malfunctioning HD500 for a new unit only to find they still have the exact same problem. Two, there is a bad batch of HD500s that have a hardware issue. This theory is supported by how random the problem is and how difficult it is for others to duplicate. The problem has been thoroughly discussed and documented on this forum and over at the TPG. Here are a few links to the highlights in this tread.

Description and Samples originally posed on TGP

More info from Joe

Workarounds:

Volume Pedal:

Meambobbo first suggested a workaround on page 26 using a studio EQ that then morphed into this volume pedal workaround which seems to work relatively well for the digital clipping issue, a little less so for the fizzy white noise issue.

1. Insert volume pedal at beginning of signal chain.
2. Push toe down so the volume pedal is at 100% max.
   
3. Reduce  the max volume % till clipping and any unwanted fizz/beehive/white  noise fades away. With my equipment I usually find the max volume %  needs to be set somewhere between 65% and 90%.
4. Create patch,  adjust the volume pedal % max up or down as working on patch till where  the amp sounds its best without clipping or noise, but the max volume %  is as high as possible.
   
5. Save preset and go jam.

The  good thing about this workaround is you still have a working volume  pedal. It just scales between 0% and whatever max % you set. The down  sides are that it may make your sound more compressed, does not seem to work as well at taming the beehive with medium gain amps like the AC30 as it does with low gain amps, you lose 1 of your efx slots, it may have other impacts on signal/sound quality and may not work on the HD300 and HD400.

Input and Mixer Setup:

There is another workaround that might help reduce this problem and hiss and other general noise problem in the HD500. Setting the input selection to Input 1: Guitar, Input 2: Variax and centering the mixer pan controls. Full details on this issue can be found in this thread.

Reporting a Bug to Line 6:

If you want Line 6 to really pay attention to this problem here are a few things you can do.

1. Go to the product feedback page and log a bug report.
2. Reply to this forum thread saying you are also having a problem.
3. Call Line 6 about your problem at (818) 575-3600 (North America) or +44 (0) 1327 302 700 (Europe).
   

I will try to update this first post if there are updates on this topic. Please let me know if you think I should add anything else to this post to clarify the issue.

Message was edited by: AZGdude